Bicycle Tutor

My seatpost is quick release and it keeps looseing and lowering when I'm riding! How can I tighten the quick release lever?

The quick release lever on the seatpost works the same as the ones on your wheels. This video (at about 0:50) demonstrates how they work.

It's not that I don't know how to use the seat locking mechsnism, it's just that the mechanism itself seems to be loose. I tighten the thumb lever but the seat is still loose!

Hmmm, have you ever changed the seatpost? Could be that the post is too small a diameter for your frame.

Quick release levers are often hard to get very tight - if anything, that is the design weakness that they have.
I'm not sure if there is a general rule, but what I do with them is to thread the lever hand-tight, and then push the lever over. If it's easy to push over and lock, then it's not tight enough. One rule I read that someone uses is that the lever needs to leave an indentation on your skin when it is done. So far, for my quick-release seat lever, has been to adjust it so it is as hard as it can be for me to push the lever over. The problem still is that the seat shifts around if I put some force on it - which means I probably need more force than what I do put on it.
Of course, quick-release levers for wheels might be different...

Sounds like either the seat post is too small or the quick release is defective. I've QR's with plastic bushings that wear and flex and don't tighten adequately. I'd try putting a simple normal bolt on there to test. Don't go crazy overtightening this, just firm. If this holds the seat, problem is probably the QR. If it still slips, probably need a new seatpost. Finally, check that the slot that runs up the back of the seat tube on the frame where it clamps the post is not closing down all the way when you tighten. If it is, that indicates wither frame damage or too small post again.