If BTC dips below x: buy $ amount (where x = percentage below current high for the past 12 hours, also based on recent volatility)
If BTC rises above x: sell $ amount (where x = percentage above current low for the past 12 hours, based on recent volatility)
I believe, without sources, that one or more market-making bots are using a similar strategy on Mt.Gox. If this works for you, then make bot on one of the trading software applications.
I will try it out for a while and see how it goes. I wish I knew how to program a bot, but I guess if I ever start trading at a high enough level I could outsource it. Not at this time though.. needs more testing + I don't deal with large enough volumes to make it worthwhile.