My answers relate specifically to coding but can be applied to anything.

1. Step away from the keyboard. Talk a walk, go for a run, talk it over with a colleague over a coffee
2. Become 10 years older! My experience has helped me enormously.
3. Use Binary chop. Split the problem into two and narrow the problem: repeat.
4. Remember Sherlock Holmes: When you have eliminated the possible, whatever remains (no matter how strange) is the answer
5. Check your test data. Well over half of my really tricky problems have been caused by faulty data and not faulty code or algorithms.