clariss, what seems to work for me is making the decision to go running when i'm not about to do it, for example when I'm at work I often decide I'll go running when I get home - it doesn't seem hard when you don't actually have to do it right now! Then when I get home I rush in, get changed and get out of the house again before I have a chance to change my mind! I think it works because I like the IDEA of going running most of the time, but if I actually have to do it right now then the excuses start flowing....
also having a goal has helped me, I actually got out of bed on Saturday morning for a 6 miler in the wind and rain, because I want to do well in my next race in November - try signing up for a couple of races in the next few months.