Here's what I've found works for me with spirals. I thinks others here will agree with this:
1. Good blade tension
2. Fast saw speed (I crank mine all the way up)
3. Cut slowly. Every movement of your piece is going to remove some wood, so be very deliberate in how you move it.
4. Learn to recognise when a blade is getting dull. Sharper blades are easier to control. If your having to fight to keep it cutting where you want, it's probably time to change it. I went through 3 dozen blades on a recent project.
5. Practice, practice, practice. As you get used to using spirals, you'll be able to go a little faster, but concentrate on precision instead of speed.
Hope this helps! Good luck!