I use a Price Book that I carry with my horde of coupons.

as rosebud said, you just write it down, then you always have it to reference. Keep as little or as much detail as you need (ie, store, date, sale, cpn, etc). I have way too many other things to remember!

I haven't been able to write in it for a few weeks (since I broke my arm), but it's still great reference...especially as we moved to a new state, 3 mos ago, it's interesting to comnpare prices that way.