Use the get Get two of one item glitch under a block that holds a Yoshi. Yoshi will pop out jitterishly and you will here two Yoshi noises. When you jump on Yoshi, you will hear two Yoshi-getting-on noises. Now, on the visible Yoshi, walk somewhere and hop off. Jump over to where Yoshi used to be. You will land on an invisble Yoshi (you should hear the Yoshi-boarding noise). Mario will look like he's walking around normally and the music will still be the same. But, if you push A you will jump up really high. And, if you stand on a block (like the ? blocks or the information blocks), pushing A will often make you warp under them. You have to get back on the invisible Yoshi after pushing A, though. He can be found back in his original Yoshi-sprouting spot or where you left him but he's often by the visible Yoshi.
We've tried this glitch so far in Yoshi's Island 2 and Chocolate Island 5. In Yoshi's Island, use a koopa shell and the p-switch at the end of the level. There are two yoshi places in the level, so you can experiment a bit. In Chocolate Island, you should use the p-switch by the square of blocks and the koopa shell by the areas of water further in the level. The Yoshi is in the ? block at the top of the square by the p-switch. Be sure not to hit the switch and kill yourself... And, jumping off the invisible Yoshi by the squared-in spinys can be sometimes dangerous because you sometimes slip through the blocks and get hit by them. Experiment and have fun.
A Youtube video of the glitch can be seen here.