I've just posted something here that I put up at Goodreads last month:
Short version: I use the website eReaderIQ to email me whenever Amazon ebooks I want go on sale. It actually strikes me as odd that Amazon doesn't have the ability to do this for its users, because it definitely has caused me to buy more ebooks. But since most of those ebooks are priced lower than $4 - well, it's been really great to have those notifications. And most of those really low prices are gone within 24 hours. (I also recommend this for non-Amazon users if your usual ebook store price-matches, or just to know when particular authors you like have Amazon sales that might be mirrored at other sites.)
Note: don't remember who recommended this to you if you suddenly find yourself buying more ebooks than you'd planned. I was never here, I don't exist, this is not my fault!
[Hmmm. I'm going to have to look into changing the font on pages linked on that sidebar, because again, crazy huge font.]