It's kinda hard to encourage trading. The best way to get people to trade is to make a trade yourself, get the ball rolling so to speak. Generally speaking I am usually the first one to make a trade in our league every year, and currently I have made the only two trades this season. I usually hear about the offers between other teams really picking up after I make a trade. I guess it's human nature... people see a trade, and they will always decide that one side did better than the other and wish that they could have gotten that "great deal" for themselves.
Alot of times people will want to trade but just assume that no one will accept thier trades. If I were you I would make alot of decent offers to just about everyone in the league. Don't screw your team up, but see what a person needs and make them a good offer. I've probably offered 15 trades and came away with 2. Now is the time to do it, as people will get frustrated with players in the first month and not willing to wait for them to come around. You'll be surprised how well you can do. I offered Rollins for Beltran and got Braun for Rollins (manager was freaking out about Braun's slow start); and I offered Ordonez, Oswalt, and Kazmir for Peavy and got Peavy for Carlos Pena and Wainwright (Pena being hot right now), just by playing up to the fears and frustrations of impatient owners. Once the other managers see you making some good deals, they will want in on the action. Regardless of who gets the better end of the deal, other owners will be encouraged by seeing a trade go through that they think they could have gotten done themselves.