Instead of focusing on keeping your butt in the saddle, concentrate on other things that will improve your riding. For example, think about your feet & the amount of weight you are carrying on them. Do you put too much weight in your stirrups or not enough weight? Too much weight in your stirrups will cause your leg to be in the wrong position. Not enough weight in your stirrups will cause you to lose the stirrup and you will become unbalanced.
So the next time you go riding, play a little game with yourself. Really exaggerate carrying your weight in your feet for awhile. Then do just the opposite - see how light you can be in your stirrups. Start out doing this at the walk and trot until you are prepared for this to be your only focus when you move up to the canter. When you can readily adjust the amount of weight you carry in your feet, then move on to another focus - like leg position, keeping your knee soft and bent in line with proper riding position (shoulder, hip, heel). Don't focus on your butt, because my guess is that your butt isn't the problem!