Knowing when and how to trade is very important. It can also be very difficult to be a successful at trading. Most of the time if you're in a good league where people know their stuff, you're not going to be able to rip anyone off. Its a bit insulting to propose a ridiculous trade to someone. Trading can be very beneficial even if its completely even.
Say if you need a backup running back. You are probably going to want to trade away one of your bench WR or QB for a backup RB. Look at the possible people to trade with and see what they need. Look for weaknesses in their team and then tell them how if we traded these players both of our teams would be better. It is great to send a note along with the trade of why you think it is a good idea. Most trades only happen with the low tier players.
Be prepared to get turned down on trades and don't use it as a major source for your players to fill injuries and bye weeks, that's the waiver wires job.