Rapid fire:
Hold CTRL to keep firing all the time.
Power-ups:
The power-up increase is shared between all weapons, so always collect power-ups when available. Then, wait for a favored power-up to switch back to the weapon you like using the best. Note: Use your super bombs conservatively.
Super bombs:
Only use super bombs when you are about to be hit in order to avoid damage. They are also effective when used to take out Bosses quickly.