Pretty loaded team there, champ. How many guys in the League? When looking at your team, I'd divide your players up into groups. Untouchable guys. Very tough-to-get guys. And then willing-to-move guys. Check the standings and see which categories you're struggling to win. Then find the guys in the last bracket and see which ones aren't helping you in those, then find free agents are guys in the trading market who will help fulfill that. When I look at your team I would definitely hang on to: Votto, Granderson, Hamilton, Trout, Cain, and Strasburg. Those would be my guys in the untouchable or nearly untouchable. You might want to consider moving a guy like Mile or McDonald (guys who have probably their highest value right now) or a big name like Lincecum (who may end up turning it around, you never know).
All in all, I'm surprised your team is struggling. Just find the categories you struggle in and try to make minor moves to improve those.