I'm the commish of a 3rd year keeper league with 20 teams and 25 man rosters. Over the past couple of years, five of the 20 managers have really taken off leaving a great divide. All managers outside the top five realize they have no chance, but for the most part have not totally given up.
So to spice up the rest of the season, I have started a competition with prizes based on who has the greatest points increase over the next one month and for the rest of the season. I decided on most points increase because it is relatively independent of actual standing position, so everyone will have a chance, especially the teams towards the bottom. Prizes are Cincy Reds tickets and Bobbleheads I've gotten from various minor league games I've been to. All stuff I got for free, but things people would be interested in anyway.
We also reset the waiver order at the all star break, and are also making waiver orders a tradeable commodity.
Additionally, we are on the verge of contracting the league to 16 teams and drafting those players to all teams outside of the top five. This will be a non-serpentine draft so the last place team will get a much needed boost. We hope this will lead to a more competitive league next year.
In the offseason we hold a 3 round prospect draft. Managers retain the rights to the drafted players until opening day at which point you need to decide whether they will be promoted to your fantasy roster or released to free agency.
Just some ideas for you, I hope they help.
As for trades getting out of hand, there's a new site going to be launched by next season I plan on signing up for since trade reviews are always controversial. http://www.flbtradereview.com