Not sure if you mean long term (as in design-level issues) or short term (trying to hack through a particular method), but in either case one of the best things

Not sure if you mean long term (as in design-level issues) or short term (trying to hack through a particular method), but in either case one of the best things you can do is turn off from actively thinking about the problem. If you're banging your head against the wall, give yourself permission to walk away and let your unconscious process and work through the information. You will often be surprised to see the mountain leveled when you return. One way of doing this is taking a long walk as a mid-day break.