Dude, on grass you won't get as much lift. I made the same mistake as a kid. Basically you've got too much holding you down and you can't get a good bounce going. What you need to do is pretty much get on something hard so you can bounce off it, get the nose up high and jump your back foot as you hit the ground.

I have to agree with LicensedtoWill. I know it's easier to do it on the grass, but trust me, it's not worth it... your wheels sink into the ground, you lose pop, and doing a trick on the grass and on cement have very different feels to it, so if you learn a trick on the grass you'll have to relearn it on cement. I also recommend learning all tricks while moving, because again, tricks while moving and not moving are different ballparks... while moving you have to factor in momentum and what have you. If you *absolutely* must learn on grass, at least put one half of your skateboard on grass and the other half on cement until you learn the basics of the trick (for example, if you're trying to learn a kickflip, learn how to get the rotation half on the grass, then once you've got the rotation down start trying to learn it on cement and moving). As for the ollies... try improving your drag. You should be dragging it a bit upwards (not flat across or you'll kick your board down and lose height). Before you ollie your front foot should be in the middle (or even further towards the tail for more height), then drag it ALL the way to the front bolts before levelling it out for maximum height. Also, make sure your pop is right (if you don't know this is when you push down on the tail to bring the nose up in the air). You should never, ever pop with a flat foot (ie. your foot shouldn't be flat on the tail when the tail smacks the pavement, instead, it should be already rising into the air with the toes pointed at the tail if that makes sense); try to jump off the tail, so that *JUST* as the tail smacks the pavement you should be flying up in the air and lifting your feet as high as possible (seriously, try to bring it up to your chest, the board will come with you if you do it right) for maximum height and so you don't stop the board from gaining more height with your feet. The pop should be a fast, snapping (though not necessarily hard) movement which will make the tail bounce off the ground and this bounce in itself will bring the board up a bit... wait for the board to bounce first so it's already in the air when you start dragging for even more height.