My knowledge came 100% from here for the first couple years of serious collecting. I was on here for a year or year and a half before I ever stepped foot into a coinshop. Then I just started looking at my coins in hand, for a while I did alot of posting my collection on here and asking opinions and comparing it to what I graded it as.
Looking at tons of coins, both in hand at shows and online in auction archives. Also, buying lots of coins of the same series and comparing them with one another is a big help. I've done that with $5 Indian Gold (a very tough series to grade), but I'm actually getting pretty good at it since I've owned several pieces and have looked at hundreds of them in auctions and photos of hundreds more in auction archives. The problem is, as mentioned above, the third party graders can be somewhat inconsistent in their grading of coins, especially in a tough series like Indian Gold. Practice, practice, practice. For Indian Cents, I read Rick Snow's grading guide which can be found on his website at http://www.indiancent.com/ Awesome resource. I also built my own grading set of Indian Cents so that I could get a general feel of the grading my studying examples graded by a third party grading service. I did a thread of that set here: http://www.cointalk.com/t59777/#post631423
Buy a copy of the ANA grading standards, look at thousands of coins, and when you are through, like me you will still have much to learn about grading coins in todays market.