Good advice so far. Especially about which end of a 2-for-1 you typically want to be on ...
I do disagree with one thing. Sometimes it's okay to overpay a bit. The cases I'm thinking of are cases where you end up with a player in the top two (or whatever) at their position, but give up two good players that together may be worth more. It's worth doing if you have a deep enough team that you'll benefit overall. One example might be if you have a solid player that's riding your bench or coming off the DL, e.g., you can trade away A. Ramirez because Chavez is back. Or in cases where a player you picked up on the cheap is performing well enough to allow you to move a more highly regarded player, e.g., perhaps Jason Bay allows you to move Matsui.
It has the added bonus of helping make the trade more attractive (thus more likely to be accepted), of benefitting both teams, and of making you appear to be a desirable trading partner to the rest of the league.
"The game has a cleanness. If you do a good job, the numbers say so. You don't have to ask anyone or play politics. You don't have to wait for the reviews." - Sandy Koufax