If you use a rocket, there is excactly 1 frame in the animation on which you can press B again and activate another rocket. You will fly 2 times then. With this trick you can fly with 3 rockets and reach very high spots.

This can save you like 5 seconds and more in the run. It's very hard to master, but it's defenitly possible for humans.

If you fly the rocket, wait until the last sprite of the flying animation. The sprite will be visible for 2 (or 3) frames. One of this frames is the one when you have to press B.