Update: I was unfortunately not aware of Shamus Young's severe criticism of Fallout 3 available here to link in the original piece and I regret that. It dovetails rather nicely with what I've written and it's much better executed than my piece. I strongly recommend anyone...
Don't always go for the obvious colour. Sometimes, it's better to think first.
Take as much time as you want - there's no limit.
Look for squares on their own, for these are the toughest to clear and should be a priority.