I've found it's a lot easier to avoid writer's block if I plan my whole story out from the beginning. I brainstorm and outline, until I know exactly where I plan to end up by part 37, even if I'm only on part 1. Do I need to stick to this outline? No. But it's helpful to have a roadmap. That way if I veer off course for some reason, I can always look and see which intersection to take to get back if I need it, and it helps me avoid writing myself into a hole.
If I get stuck despite an outline, I tend to take frequent breaks and do other unrelated but relaxing things. Often, just standing in the shower and letting my mind go will bring some awesome idea that I can't wait to put down on paper.
If all that fails, one thing I like to do is go to a crowded place and people watch. Listen to the conversations, and write down the interesting ones. You won't have all the details or the context, so some things may sound incredibly peculiar. Write a new story to explain them, or figure out how to work them in as passersby in your current story. I find that adding bystanders unrelated to the current plot can often add a bit of atmosphere to a setting that may have otherwise seemed flat.
