I would say the best way to avoid writer's block is to plot your book out beforehand. That said, you don't have to figure out every detail, but have an idea of where the story is going and map out certain plot points that you want to hit. This way, when you get to a point where you're running out of steam, you can pull out said map, look it over, and remind yourself of where the story is going.
Another option is to write a drabble with one or two of your characters. Don't put pressure on yourself to make it good or make it part of the novel. Just start writing-it can be silly or you can even start with a situation that you experienced or saw on TV and plug your characters into it. Think of how they might react, etc.
Or you can re-read the first 20,000 words to remind yourself why you enjoyed writing the story/characters so much.
If all else fails, shelf the project for a short period of time to give yourself a break from the project and time to start missing the characters and storyline. Don't stop writing altogether, however-if you have other projects running, work on them, or just continue with your words-per-day goal and keep at it, even if everything you right is a silly drabble.
Hope this helps!