I've become quite a pro on pulling a wheelie on my rocking cow and I'd like to share some of my tips with ya'all.
First off is the distractionary tactic. Your parents will probably have a coronary if they see you standing up on your ride so I suggest a good "oooh look over there!" before attempting. Handy hint: Moms love things like "can you believe she is wearing THAT out in public?" and if Dad is watching you try something like "can you believe that is ALL she's wearing out in public?" A subtle distinction makes all the difference in the world.
Once you have them good and distracted, you can initiate your standing sequence. Make sure to shift your weight forward, as your diaper will offset your weight and make you bottom-heavy, particularly if you are in need of a changing.
If you are discovered standing one of two things will happen: said adults will either:
a) Shriek, wring their hands, and run over to save you from imminent fallage
OR
b) Run to get the camera to capture imminent fallage on film for all posterity
In the case of a), the recommended reaction is your best "baby" face. Apologetic, raised eyebrows, a look of confusion and maybe a little drool. When in doubt, claim ignorance. I recommend something alone these lines:
If b) has occurred, rejoice! You got the pratfall parents in the grownup lottery. The only downside is they're probably recording all of your embarrassing moments and posting them on the Internet as we speak. Look ma, on one leg!