This is still just a very brief introduction that I'll hopefully continue to add to. If you have a good suggestion, please add it to the comments.
This is mostly as it relates to foraging for wild edible and medicinal plants. I've been learning so much from my mentor Jackie Dill. Go to her site okwildcrafting.com and join the Facebook group and/or yahoo group. Everyone helps each other learn.
First off, identifying plants is easiest when they are in bloom. Many of the field guides are sorted by flower color. This is often the time to harvest many medicinal herbs. Plants with edible leaves often become bitter when they are blooming though, but it gives you a chance to learn about the plant so you can identify it when it's young and tender.
If you can, buy some field guides to edible and medicinal plants and read them. I mean really read them! Flip through them occasionally and look at the pictures. It is vital that you learn enough about plant structures to know if you've got the right plant or not. Just looking a pictures can get you started, but you have to read the descriptions and be certain. Here's a good basic guide to add to your knowledge: http://www.kansasnativeplantsociety.org/documents/PlantIdentificationResources.pdf
Here in Oklahoma you have to buy the ones for the eastern and the western US because we have both portions of both sets of plants. If you can go on nature walks with an experienced forager or even alone, you'll get better at looking at plants and what key features tell you what it might be. I can generally guess which family a plant is in and then narrow it down with a field guide or one of the websites with good guides. After enough experience, you may be able to tell what family or even genus a plant is in just from looking. This will make identification down to the species much easier.
The internet is also a great place to look for information. If you already know the name of a plant you can google it. Google also has a link to look at images, which is often useful if you're googling the plants common name and don't yet know the botanical name. Beware of using common names only. A common name like nettles, for instance can refer to both a nutritious weed and a toxic one.
Also be very careful if plants do not match all the characteristics. The wild carrots and poison hemlock look a whole lot alike.
For browsing pictures to look for matches here are some good sites:
http://okprairie.com/Flowers.htm One page of common wildflowers, with pictures, by date of bloom.
http://www.kswildflower.org/ Kansas Wildflowers & Grasses, listed by color and time of flowering.
http://www.missouriplants.com/ - Missouri plants - listed by color and by alternate or opposite leaves.
http://www.biosurvey.ou.edu/ Oklahoma Biosurvey - has a searchable database but it's mostly for really fine tuning which species you might have and not so much for early identification.
http://www.swcoloradowildflowers.com/index.htm more pictures mostly west of us.
http://www.ediblewildfood.com/edible-weeds.aspx another good one with pictures of edible stuff
I'll add more as I think of things.