For the next month, United is offering double miles if you register, buy tickets and complete your travel between Oct 21 and Nov 21 2013. Valid on United or United Express within the 48 continental US (sorry, Hawaii and Alaska are out, they didnt want to give away THAT many double miles!). You have to register to be eligible.
It doesnt work for existing tickets and it also doesnt work for really low fares purchased, but if you are buying tickets now, this will be a good deal for you.
If you want to read more about this here is the link to flyertalk forum.