You must have an absolutely 100% reliable fuel system than can supply enough fuel for the NOS without dropping pressure, vapor locking, etc. A "T" behind your stock mechanical fuel pump is not going to cut it with a 150 kit.
You need to pull out 2 degrees of timing for every 50 hp if you're running pump gas. It can be done manually, but performance off the bottle will suffer, or you can do it via a seperate or built in automatic timing retard.
A fuel pressure cut-off switch is really mandatory IMHO. If you go lean, you WILL burn a hole in a piston.
Don't ever, ever hit the rev limiter while on the bottle. Your intake manifold may be found in the next state. Most people use a "window switch" to prevent this form ever happening.
That's all I can think of for now. :)