Method 1
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!

Method 2
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.