Normally, when you attempt a 'trip' and fail you are yourself tripped, but if you take the 'Improved Trip' feat you have a better chance to remain standing if you fail. You also get a +4 (i think) to your Str for trip attack checks.
Try tripping with Fruella, she's a Str of 16 and the Improved feat. She's very successful most of the time, even trips giants after a successful hit with 'Ray of enfeeblement'.
Hope this helps
