Spending a day or two confined to the library, looking for ways to solve your problem, and reading anything that seems even slightly relevant. Reviews can inspire new ideas.
Forget your current approach for the time being, and look at the problem as if you had never seen it before. Generating new ideas can help solve old problems.
Write up your progress to date (WHAT? Write something that nobody will read??) The process of organizing your experimental results in order to present them in writing can reveal new research directions. It gives you a broader perspective on the project.