Having a context for the problem or solution helps to solve this problem. I have used MindMaps for abstract things so that the context that you are working in can be quickly loaded back in your brain. For code, I have used the idea of clubbing together files related to a problem as a quick way to get the context back in your head. Take a look at Eclipse Mylyn at http://www.eclipse.org/mylyn/. It let's you save context i.e. files, resources etc. for the bug/feature/enhancement that you are working on. You can also save this context in your defect tracking system as attachment and share it with your co-workers. Invaluable.