Well, depending on the kind of story I'm writing, I'll throw in a new character to change things up for a while.
Considering I write mostly horror, having a filler character get offed is another way I keep interested.
Aside from that: listening to quiet music in the background, brainstorming plot points, or doing drawings of my characters helps. Pretty much anything that won't get me interested in another topic - but keep me slightly - distracted works.