This post on pacing at Writers Helping Writers gives hints on handling backstory and internal monologue without overly slowing down your story.
The first example is taken out of context. It didn’t make sense when I first read it, but I especially like the second one about adding bits of action while characters think or talk. This technique fools the reader into thinking something is happening when it’s really not and helps avoid the “talking heads” sysdrome.
Think small action like shuffling papers or stirring soup. I need to include bits like that more often, not only for pacing but to help readers picture the scene. What small actions have you used?