Try several different set ups, and then find what works for your benching style. Try spreading your legs. Try bringing them in and squeezing the bench between them. Try flatfooted, and try a big arch with your feet behind you (I do this, but I also get my heels on the floor). If you bench flat-footed, try this little trick: lift up your big toe. This helps a lot of lifters keep their ass down, and also helps them get tight. Keep experimenting and trying new things. Who knows, in the process you may come up with an original idea that works for you.
Spread your feet wider. Pull your feet back more or do both
make sure your quads are stretched right out so your knees are pointing down at the floor and get tbose feet BACK. set them out from the bench a bit wider than you think you oughta and pull them back far enough that your heels can't touch the floor. when benching, drive the heels down and TRY to touch the floor, which should be impossible.
one size dont fit all.