Check out Headliner.fm, a fresh start up which allows for bands to work with each other in cross promotion. The idea is simple: Biffy Clyro opened for Muse in their European Tour this past spring. You can imagine the massive uplift they got out of any shout out Muse did about them to their own (massive) fan base.
What headliner.fm allows is for bands who don´t necessarily know each other but like each other´s work, to easily and effectively put in a good word for a new album release, a new concert date, etc. Many bands work together anyway, play same venues and try to create some kind of underground movement – and this is a great tool to capitalize on these relationships to everyone´s gain. It is also a great way to build a relationship with your fans, so they understand what music you like other than your own, what gigs you are into, etc.
You can do the now standard sync of you MySpace/Facebook/Twitter accounts and oh, they also take care of that little part of the equation: implementation. Don´t worry about forgetting to post on behalf of your mate´s band, as headliner.fm does it for you