Shop It To Me is a great service if you know your clothing brands and like buying clothing on sale.
Select your favorite brands and sizes and they basically check every store on the web (Nordstrom, Macys, Shopbop Anthropologie, etc...) and will give you a daily summary when any new item goes on sale in your size.
You can either get an alert via an email or their iOS app (which I use).
Great service overall and definitely recommended!