I can do it without too much trouble but on my last bike I moved my seat back an inch or so and that small change made it harder, so I think it's important to keep some weight on the front wheel. Sitting up totally straight or leaning back is harder than maintaining a slight lean forward too...

My saddle isn't terribly comfortable when I sit up straight so I don't do it a whole lot but periodically it's fun to do and rests my hands.