This is most easily performed in World 1-1. Grab a POW block, and carry it to the set of logs. Wait for a log to bog over the waterfall, then throw the POW block so that it hits solid ground. For some odd reason, this will cause the log to fly high into the air. Strange, eh?
-Double Jump - 2 Methods
Use any character and run right toward an enemy at full speed. Just a split second before you think you're about to get hit, jump! With good timing you'll be able to jump yet again in midair!
This is yet another way to jump in midair. Stand on the ground while you have 3 or 4 life units and let an enemy run right into you. You'll lose a life unit, but you'll be able to jump in mid-air.