In my oppinion, its not painful. I used to stand on my horses back and trot around..I have no idea how I did it.
But it was fun. If you trust your horse and he/she trusts you, just (using a saddle at first) slowly work your self up..try on you knees first, then kind of stand with your knees bent and just work your way up. If you feel off balance try just holding the reins or grab a piece of mane. Once you feel comfortable, you should be ready to go! Using a bigger saddle is a lot easier then doing it bareback considering you will most likely slip bareback. After I did it the first time (on a trail ride! LOL) I was trotting down the trail, and he almost broke to a canter. It's a lot of fun once you get your balance point.