General Multiplayer Tricks
In levels with grapple hooks (Shooting Gallery and Sidehopper Station), you can reach the grapple hook (and therefore the upper level) by standing under them a little to the front/back, looking up, double jumping. This puts you just within grapple's reach, and is good for nabbing the super missiles that are under some of the grapple hooks or quickly sneaking to the upper level.
Ummeiko (I can't really take credit since it's in the Players guide)
If you lay a morph ball bomb under an opponent, it will detonate immediately, instead of the normal delay that you have when laying a bomb.
No matter where you drop a Morph Ball Bomb, it will immediately explode if someone touches it. Thus, dropping Morph Ball Bombs behind oneself while fleeing will not require timing to have the bombs explode at the right time, they will explode when someone touches them.
Also you can roll behind a standing opponent and continually bomb them until they realize what's hitting them. You can also use this to land on their head.
added Dark Samus
try sneaking up behind someone and fire a charged Dark beam then fire missles as fast as you can if you got em', Dark beam or annihilator beam if you don't