Put something in the title that gets people's attention such as "motivated seller", "must sell", or "open to offers."
Be completely honest about the condition of the bike and write more than one sentence when describing it. There is nothing worse than going to look at a bike described as perfect that is junk in reality. Make sure to highlight the good things such as new tires or brakes. An oil change and fresh tune up are also good selling points. If it is on the road make sure it has a current state inspection.
Take lots of good quality pictures and make sure to wash and dry the bike before taking pictures.