(1) Converse with your leaguemates. NFL, fantasy, gardening, golf, whatever. Develop a rapport that shows that you are a good guy. This will build trust, and at times, you might get some info on how they value a particular player(s) or even have a discussion about a trade.
(2) Figure out how other teams value players. Do they use rankings? Dynasty start ups ADP? Last year's numbers? The Shark Pool? This will allow you to identify and set value on both sides of the equation. It will also give you an idea of which guys are untouchable.
(3) Figure out their goals/vision (win now, balance, young talent, build through draft etc.) and craft offers that take those goals into account. State what you believe as their goal/vision in the trade offer to make sure you are correct. If you are wrong and get corrected, well now you know, and that is valuable info.
(4) Be polite and respectful - Don't be a jerk. Sometimes, you will get it wrong, the other owner will have an unreasonable valuation, or a trade offer will be dismissed out of hand because he's worried about getting fleeced . Don't pester the other guy for an explanation, get argumentative, or demand a counter offer. Just let it go and tell the other guy thanks, but it isn't going to work out.
(5) Remember that it is just a game.