I use a Schroth Rallye 4. Good investment that keeps your butt planted in the seat, so you can focus on steering inputs rather than hanging on.
A good hint that I learned early on (before getting a harness) is to put your seat as far forward as you are comfortable with the seat back as vertical as possible, then use your left foot to force your behind into the seat hard. That way, you're firmly planted *and* you're not thinking about grabbing the clutch when you're in a turnaround.
