However, if keeping your foes off your friends, the following list has more mobility and stickyness. Follow the red asterisx.
Taking 3 levels of Knight (PHB2) to combine Bulwark of Defense with a martial adept feat (ToB) to gain Step of the Wind* (setting sun stance) will greatly improve any spiked chain* tripping build you might have in mind, both to succeed in tripping and avoid being tripped. It will also make you very "sticky" and hard to get around even when the enemies are not lying prone. (Plus, Knights have a good Will save and a d12 hit dice.)
Using weapon groups varient in UA, you could also avoid the need for taking exotic weapon proficency and go strait for the above feat, followed by Expertise, Improved Trip*, Combat Reflexes and all the rest (Weapon Finesse, if applicable)
Improved Initative can help you advance toward your foes in order to threaten them before they begin their turns (allowing your Bulwark of Defense to take effect) keeping them from tumbling/moving past you and advancing upon your friends, possibly pulling them to the ground in a spot farther from your friends as well.
A 4th level Knight can move at full speed in medium armor, adding to your ability to intercept your foes in that regard. Plus you gain the ability to provoke enemies into attacking you in preference to your friends. Hopefully by then mithril armor is an option, depending on what your dexterity modifier might be.
At 6th level you can begin opting to take damage that adjacent alllies would have otherwise recieved, and also you impose further tumbling penalties upon those foes that you threaten. Once you have 6 levels of Knight, and having only 4 levels left to spend, I'd condier mutliclassing into something else. In addition, 6th level is golden for base score progression. After that, I would consider the following feats:
Stand Still (ExpPsiHB)
Improved Expertise (CW)
Allied Defense (Shining South)
If you are human, then Jotunbrud* (RoF) as a first level only feat would be a good choice for your tripping build, allowing you to count as being one size larger for the purpose of opposed checks, when its advantageous to you.
You can also add the +1 Sweeping ability* (MoF) to your weapon for an additional +4 bonus on opposed strength checks made as part of a trip attempt. Giving you a grand total of +18 on trip attempts before applying strength, by following the red asterisx.