I came across an interesting article on The Garter Girl about asking for your garter back. I can imagine many brides want to keep their garter, so how do you deal with the whole garter toss tradition? Ask for it back? According to The Garter Girl blog that is a perfectly acceptable request, but it might be easier to avoid the issue all together. Throwing a separate garter sounds like a great solution, and I have even seen an interesting prank at one wedding.
Good luck holding onto that garter girls.