With a jet bumper, I make sure the contacts are clean, then that the point coming down from the bumper skirt is exactly central in the 'spoon'. It's important to have the long blade of the switch tensioned properly against the spoon/point; not so stiff that too much effort is needed for a ball to move them, but not loose enough for vibration to have an effect. Then adjust the backing of the shorter blade to set the gap between the contacts. Make this as small as is sensible, particularly with 'Special' or 'Direct' switches, which will lock-on the coil if they stick closed. Ideally, full deflection of the bumper skirt should see the contacts close, and then over-travel or 'wipe' to help keep the contacts clean.
I sometimes changed the game adjustments where bumper hits were tallied, such as Raise The Dead, jackpot countdown on creature from black lagoon, or Super Jets, if I felt that the features were programmed for the frankly ****-poor assembly line bumper adjustment. Straight Jacket Escape could be completed in one bumper sequence after adjustment, whereas it couldn't out of the box.
An operator that I bought a Firepower from said that he used a PTFE spray in the hollow of the plastic spoon. The effectiveness of this may partly explain why it was a few days before I noticed that one of the four bumpers wasn't actually working - the three remaining ones were incredibly sensitive.