Wildcrafting is the practice of collecting plants from the wild for food, medicine or other purposes.  I use the term "wildcrafting" and not just "foraging" because it generally means that ethical considerations are taken when choosing the plant, the amount taken, and where it's taken from.  The term does make me feel a little hoity-toity in the granola extreme, however.  Use whatever term makes you comfortable, but please take into consideration not just your own health but the health of the plant, the ecosystem you're taking it from and, you know, the earth in general.

Most of us have been told from a pretty young age not to eat anything wild from the outside.  We're told it's yucky, poisonous, or dirty.  This is usually from caring elders who don't want to see their younglings all laid up with diarrhea or worse.  It takes a little practice to get over this semi-false survival instinct.  I find that I'm still a little hesitant when I cook something new up. I have thoughts like, "You just got this off the ground in the woods.  What are you crazy?"  But there are measures you can take to reassure yourself of at least the safety of a plant, so that you can spend your energy discovering the taste it does have rather than the lurking poison you think it might have. Here are some basic ground rules.

1. If you don't know for sure what it is DO NOT EAT IT.  If you don't know what it is but see an animal eat it do not assume it's safe for you. 
 2.  If it taste really bad, I mean really bad, don't eat it.  This usually means it's not safe for people.  We do have a built in ability to eat without killing ourselves.  Your ancestors worked for generations making sure that you would not run out and eat something that's going to kill you.  If you have identified a plant, harvested the correct portion, prepared and cooked  it the way you are supposed to, get your fork, take a bite and think, "This is disgusting, I wouldn't eat this in any post-apocalyptic future," put your fork down.  This is your ancestral knowledge speaking, it's like your conscious but more primitive--like if your conscious is a cricket with a top hat, your ancestral knowledge is that owl form The Secret of NIMH. 
Ancestral Knowledge
You tell me who you'd rather ignore.  The choice is yours. 

This can be a little tricky at first when you try to tell the difference between understanding why some random wild edible wasn't chosen to be cultivated, and poison.  I have also noticed that enthusiastic foragers will proclaim something as "delicious!" or say that it "tastes just like asparagus!" and this simply isn't the case. Most of us have taste buds worn down by foods--even veggies--that are pretty sweet.  I have found that most wild greens and vegetables are just more bitter than the greens and veggies I'm used to.  But this sort of bitter-perhaps-not-delicious flavor is not the "put down your fork" flavor you should be looking out for.  You will know that flavor if you taste it.
3. Make sure your wild goods are free of bugs and poop.  Contrary to popular belief, this won't make it any sweeter.  Here you see some recently hatched beetles and below that signs of a leaf borer. 
 Sorry, no pictures of poop, I bet you know what you're looking for there. 
4.  Make sure the land where you're foraging is free of toxins and hazardous chemicals.  Huh? How will I know that?  I know, right?  I feel you, it's hard to tell.  Here's what I say.  Don't harvest from right off the side of the road.  That little patch of grass between the sidewalk and the road is off limits for you.  When you see something you want to eat, get a few yards from the road, if what you want to eat is growing there, it's probably ok.  If the plants show any sign of distress--yellow leaves, brown spots, some of the plant is dead, just find somewhere else to go.   That being said, I will still harvest things like acorns off the side of the road.  I have a lot of faith in that huge oak to cleans the bad stuff right out of those nuts.  But use your own judgment. 
5. Know your local history.  If you were in the woods behind the house I grew up in and were tromping around trying to forage for wild edibles, it would be good to know that the company that owned that area previously poured copious amounts of toxic chemicals into the ground and that the water, soil and more than likely the air is all poison. In this particular example you would have some sense of this from the chain link fence, the "No Trespassing" signs and the workers in bio hazard suits.  Usually, it's not quite so clear, indeed the woods behind my childhood home weren't guarded quite the way it is now and we played up there from dawn 'til dusk.  While you can't know where every house was and where every Joe who lived in that house decided to dump his motor oil, you can find out if those woods were the property of a weapons factory.  I found a great spot for picking blueberries, but wanted to make sure that before it was a pine forest it wasn't something toxic.  I stopped by our historical society and asked. They informed me it was a farm previously, so I feel pretty safe collecting my year's worth.
6. Take it easy.   Don't stress, there isn't much you can eat that will kill you.  Really, besides water hemlock, which can kill you, most things will just give you diarrhea.  While unpleasant, it is unlikely if you stick to rules #1 and #2.  Another thing I mean by "take it easy" is you should test foods out in small doses.  Everyone is different, you may have a reaction to something.  If you have a little bit of something and have a little reaction, it stands to reason that a lot would mean an increased reaction and so you should avoid that food in the future. 

With these things in mind you should be able to keep yourself safe enough, but what about the environment?  There are a few other guidelines to keep in mind when you are out there communing with nature, enjoying the harvest, and trying new things. 

1. Don't eat it all.  Seriously, share.  I have read more than once the proposed guideline of 10% of whatever it is you're harvesting.  Obviously this too also depends on what you're harvesting.  If you're taking about a root of something and will be in effect killing an entire plant make sure it is from a patch of plants that is thriving.  You would then only harvest 10% of the healthy, thriving patch of these plants.  I have been dying to make candy from sassafras root, but have been unable to bring myself to pull up trees in order to do this.  They do grow back from tiny pieces of root, and they grow in patches, a large tree sprouting smaller ones, but when I put my hands around the trunk of a small tree and go to pull, I find I am incapable.  With other plants, it wouldn't matter in the least if you were to uproot the whole thing.  A a patch of soapwort probably wouldn't care if you pulled up 50% of it, it would just grow right back the next year, no sweat.  Also, if you're harvesting berries, you're not actually doing anything to the plant itself, simply taking the seeds for future plants. I will happily harvest a lot of berries out from a given patch, and will sometimes have to be reminded by my screaming children to, "Save some for the animals!"   They don't necessarily eat the same things that we do and they can eat things we can't, but of course, use your discretion.  But I guess you would also want to leave some for other potential berry pickers so they too can experience the joy of homemade wild jam.  No matter how much you take you can, off course, scatter the seeds after you've juiced the berries.  No harm, no foul. 
2. Move around.  You will probably find a great spot and return to it year after year, but pay attention to the growth of that plant.  If your harvesting seems to be hurting its life and progress, go somewhere else for a while.  On a similar note, if it looks like other wildcrafters are hitting up that spot, go somewhere else (and leave a note and be friends!)
3. Don't pick endangered plants!!  Know a little information about what it is you're looking for.  There are many plants that have been picked nearly to extinction for their edible and medicinal qualities.  Don't pick these ones.  None of them taste so wonderful or are filled with such healing qualities that you can't find your pleasure somewhere else.  The USDA has a great site listing the endangered plants of North America.  
 Goldenseal has been picked almost to extinction because of the price it's root will fetch. 
4. Pick the heck out of invasive plants.  Some of the plants above are endangered because their habitat is being taken over by invasive weeds that were brought over here on purpose or hitched a ride on the trouser leg of a colonist.  These are guilt-free harvesting opportunities. If you come across this beauty,
harvest all the lycopene-packed berries you can manage.  This is an Autumn Olive and the bushes make so many berries that you could get a gallon off of one good-sized bush.  Just make sure you dispose of the seeds properly.  I know someone who puts them in their compost, I prefer to burn things like this.  It just makes me sleep better at night.  The USDA also has a spot to look for invasive weeds here.  Don't just rely on your state's list, sometimes plants that should be on the list are kept off for one reason or another, so check the states around you and the federal list also. 

That's all for the "Save the Earth" lecture, I think.  

Here are some links to people who know more than I do:

Russ Cohen
Russ's book, Wild Plants I have Known, and Eaten, has some really fun recipes and info in it, but it also has a great calender of the various seasons for some wild edibles in eastern Massachusetts.
His site also has a great list of resources here

Steve "Wildman" Brill
Steve's book has very detailed information on the plants and some cool recipes as well.

Here's another book list from Foraging.com 

Peterson's Guide to Edible Wild Plants was my first book on the subject and I find it very helpful.  It's a field guide, so it's great if you're just learning about identifying plants.

And of course the Peterson's Guide to Wild Medicinal Plants and Herbs is another great resource.  

If you know of any other resources that you have found helpful email me and I'll include them here.

Happy foraging!