Leave blank lines, and go on to next person's dialogue, or go to another scene. Stop your writing in the middle of scene, so you have something to come back to. Utilize your slow times for research, plotting, etc. If none of these work, try defining your own triggers. Try reading a really bad or really good book, these can inspire you to do better.
Special music can set a mood or tone and spark ideas.
I personally love old black and white movies, especially starring Jimmy Stewart. These can give you idea for conflict and plot twist. Physical activity or exercise, it gets your blood pumping and your mind jumping. And let's face it, I'd rather write than exercise any day. Don't let yourself think too much about the block, this only makes it worse.