Get a leaf or tanooki suit. Press run or alt run to swing your tail and keep holding the button. Now press down. You can't crouch. This holds true until you press jump (not alt jump - spin jumps don't count against the glitch) or release the run/alt run button - even if you grab a different powerup, as long as you're still holding the run/alt run button, the glitch is still in effect.
Yoshi021: Bug Fixed
Tested Using 1.4.2 (Patch 6)