I too find 4-leaf clovers and a regular basis (i've found at least 50 in the last 3 months). If you put them in packaging tape, then take a needle or safety pin and punch a hole extremely close but not through the clover (so that air can get in and out), they turn brown but stay flat and preserved. I've found that works the best to keep them.
I'll put it in something to keep it preserved , probably the same containers you hold comic cards in, after it's been pressed between books. I have many already preserved, but I would pick fresh with the thought that it's for someone else in mind :)