Maybe get someone to lunge you whilst cantering to get the feel and then try on your own so you have to proper feeling and a good position?
Heels down always...sit up, look over ur horses ears, keep a nice and collected canter
Try not to tense up because that can cause you to lose more balance. You can also squeeze slightly with your legs to help "grip" the saddle. Also my trainer always says to pretend that you are a wet towel and sink into the saddle.Good luck!