| When all else fails, use your standard action for Total Defense (+4 to AC) and your move action to stand up; this effectively cancels out the -4 penalty to your AC for being prone when you stand, so none of those attacks of opportunity get a "bonus" to hit you.
Or, if it's an option, go on Total Defense and don't stand up (thus, no attacks of opportunity, and effectively no penalty to your AC against any regular attacks directed at you). Wait for help to arrive.
In any case, don't despair. Improved Trip tends to be very effective at low levels, but a lot less so as you go up. | 
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