Fin keepers! If your boots are roomy, it's easy to feel as though you are going to kick your feet right out of them if there's any air in there (and you can do it, too!) Fin keepers are these Y-shaped elastic bands that you can wear over the boots that collapse the boots around your feet and make the boots much more secure. They're wonderful.
Also remember that, if you are on ascent in open water, there is NO reason not to go slightly out of trim, drop your feet and get the air up to vent. When Joe Talavera taught our Rec 2 class, he told us he's had to get behind tech divers and yank their feet down, because they were so concerned with being out of trim that they wouldn't get the gas out of their boots.
In my very limited experience, getting a set of doubles weighted and trimmed out properly takes some fiddling around and sometimes some gear changes. If you're head heavy, you may need to a) move the wing up b) move the tanks down (limited by needing to reach valves) or c) go to a lighter BP and a weight belt. I had to buy an Al backplate to get balanced in my doubles.