A good strategy I've found was suggested by Reid Burke in response to this answer:

http://reidburke.com/2012/09/27/write-code-that-works/

Your code isn't uniform, and so you shouldn't be expecting 100% code coverage on everything either.

Instead, assign to particular sections of your code levels of stability, and for the code that has the highest stability/least amount of expected future change, prioritize writing the most unit tests for those.