All the suggestions listed are perfect, you could also check your leads shield as well. cheaper leads tend to break down quickly so have alook at it. Also are you using a mike? that tends to add to the problems. Distance from the amp is the real solution if you cant get a noise gate. If the venue you play is small then get the amp up high using crates to stakk it. that is a good help and gets the sound out better over the crowd so you can turn the gain down a little to kill the feedback.