What methods do you use to hold you accountable? Do you have a set plan in place to keep you from falling off the writing wagon? If not, you may wish to explore some of the methods that have been discussed in this blog or elsewhere.
They key is to make sure that you have a system in place that does not allow you to rationalize not writing, and makes you accountable to others.
It needs to be something that kicks in quickly, within a day or two after you have fallen off the wagon.