Hi Bella!
Usually I just bookmark items and check up on them once in a while. Or use shopping carts.
I used to use savvycircle.com but they don't seem to work well and have lots of glitches.
Recently I discovered that Shopbop will e-mail you when items on your wishlist go on sale. (Of course this doesn't help much when you don't check your e-mail frequently. I have an e-mail filter for such things, but don't check it frequently, so I just missed out something I wanted)
Also, Shopstyle has a very extensive e-mail alert system you can set up here http://www.shopstyle.com/salealerts The number of brands it lists is frankly overwhelming. I tried it out for fun, and couldn't keep up.