Invest in good reference books and read them. The cost is miniscule compared to the cost of mistakes made out of ignorance. Get to know other collectors. If you're new to the hobby, you don't have to buy from or sell to them yet. Just visit them, see their collections and learn about values and condition and rarity.
Books are fine, but they provide only a framework of knowledge.
If you're new to the hobby, don't trade without getting multiple opinions. If it's a good trade now, it will be equally good after you've had a chance to learn a little more. When I first contacted Jim Frazier, I bragged to him that I was a shrewd trader. And I thought I was. He asked what they were, and I proudly told him there were two Pikie minnows and three South Bend Bass-Orenos.