High gain/high volume feedback is a bit of a problem when you don't want it, standing away from the amp, damping the strings when you aren't playing, and turning any pedals off should help. If you've tried all these and you're still getting feedback, it's time to look at other possibilities.
What amp are you using? When did you last change your pre-amp valves/have it serviced? What pedals are you using? etc, etc.
Feedback can be your friend sometimes though... :twisted: