When I was young and insecure, I practiced on a pillow. That REALLY helped! You really want to do just your bottom because it will tire you out less and you won't be lifting and dropping as much weight on him. I personally think it looks better, too.
If you have a sex toy with a base and you're comfortable using it, you can strap it down to a pillow or a chair and practice that way. You should be able to "feel" the dildo moving in and out of you...I say that because dildos often feel more "synthetic" than an actual penis, so it's easier to tell if you're actually bouncing properly.
One thing that really helps me, because I'm petite is to lean forward and put my hands on the guy's chest, or if I want to go slow and deep, I lean forward and put my hands near his shoulders while I kiss him. If he's an ass man, he'll probably love it if you face away from him and lean forward; this makes it easy to be "on top," cause you just slide back and forth. Leaning is generally great for women who like to have their g-spot stimulated.
I've also had sex with women while wearing a strap-on (erm, sorry, I guess I'm getting a little TMI here, LOL! but I want to help...), and because I was usually in the more dominant role, I would usually have them stay relatively still while I pumped MY hips up and down. This is one of my favorite positions with guys, too: I'm on top, but they do the work.
A variation that I think looks silly, but every man I've been with has loved, is when I squat over top of him, balance my hands on his chest, and move up and down with my bent legs. It's great exercise, too, kinda like doing squats. XD
You don't have to "just bounce," either; you can also grind or rotate your hips. I've also teased men by doing a bunch of shallow "bounces," then a deep one....rinse, repeat. They usually got ticked off after a while and gave me what-for. Not that I was complaining.
If you're both sitting on a bed with a headboard, or a chair or a couch, you can use the furniture to help you lift your weight up and down.