Use the
martial rogue variant for more feats. I know you said no odd races, but being big REALLY helps for tripping. I personally like Goliath with Barbarian multiclass (using
Wolf Totem to get trip for free). If you have FR books, there's a regional feat to count as being larger than you are (Jotenbrund?). At the very leas,t enlarge person frequently.
Multiclassing really does help a lot. I mentioned Barbarian, which also gives rage (and as a Goliath large size while raging), but Swordsage for the Setting Sun discipline (both in Tome of Battle) works nicely, too.
For feats, aside from Improved Trip, go for Curling Wave Strike from Stormwrack. It lets you use your free attack after a trip to instead trip a second person within reach. Consider improved disarm. If someone's prone, you effectively have a +8 on the opposed roll +4 to attack prone in melee; -4 to melee attacks for prone targets). Defensive Throw from C. Warrior is really cool if you pick up Combat Reflexes. If a foe misses you, it provokes an AoO from you that you can use to trip them.
More arguably, Whirlwhind Attack (or the similar maneuvers Mithral Tornado and Adamantine Hurricane from Tome of Battle) would allow you to trip every enemy around you in one turn, since you can replace regular attacks with trips.
Back to Swordsage, aside from all the tasty throwing maneuvers based around trip that Setting Sun offers, the level 1 stance (basically a continuous benefit) gives you a +4 bonus to trip and do certain other combat maneuvers when the enemy is on difficult terrain. even if you don't want to multiclass out of Rogue, consider picking up Mighty Throw and that stance (Step of the Wind?) with feats.
Find ways to minimize the Rogue's risk of getting tripped in response. For 600 gp (or 900 gp as a tacked on benefit to existing boots; or 1200 for a non-body slot), Magic Item Compendium has the Boots of Agile Leaping. One benefit is using dex on jump checks instead of str if it's better. The other is, with 5 ranks in balance, you can stand from prone as a swift action without provoking an AoO. There are also two skill tricks in C. Scoundrel (each usable once/encounter) to stand from prone as an immediate action or a swift action, respectively. Both also protect against the AoO. Finally, in C. Adventurer, they mention that you can use a DC 35 tumble check to stand from prone as a free action without provoking an AoO. Alternatively, just use the Prone Fighting feat (C. Warrior) and stay prone. But that feat requires Lightning Reflexes and thus is seldom worth getting.
More MIC items:
Brute Gauntlets: Very cheap to buy, gives a few charges per day to get a small benefit on str-checks made that round.
Belt of Athleticism: Also very inexpensive, as a swift action, you can take 10 on balance, tumble, and several other acrobatic/athletic skills for the round, usable at will. Once per day, you can instead activate it to take 20 on one check with one of those skills.
Tripping (I think that's the name?) weapon enhancement: +2 market price weapon mod, to get +2 on trip checks. IMO, not worth it ever, but if the rogue really wants to optimize tripping...