I've found that a lot of times there _is_ a little switch in my head. Every once in a while I'll be banging my head against a problem, but if I stop and ask myself "how would a physicist solve this?" I can figure it out.
It takes an effort for me to step back and ask myself what the problem is, rather than just sitting down and working on it.
Usually "just sitting down and working on it" works, which may mean my problems are too easy. :)