Using CDs near the beginning is nice. Once you have 5 stacks, I generally use whatever abilities I can, throwing in Rapid Shots every other ability. My heat will ramp up pretty high and then I just drop it all with the CD. You can do that 2 or so times a fight typically.
When you can't use that CD, I have my alacrity at a point where I can use Tracer + Rapid shots and that counts as 1gcd, so as soon as rapid shots is done I can use Tracer again. My heat won't go above 0 doing this. Use Rail Shot every 5 stacks you put on yourself (with 4pc its 0 heat cost) and unload whenever its procced. You will never see your heat ramp up. It will start climbing, but once you use Unload, it will typically drop back down.