How do you deal with continuous requests for cooperation?
makecheck: Opportunities really do come out of nowhere.  It's risky to leave any unexplored.But use a priority queue.  In other words, feel free to turn people down while you're busy, but don't forget who you've said "no" to.  Over time, a person's priority goes up, just like when you're fixing bugs.  If you've said "no" 3 times to a lunch, eventually you should say "yes" even if you are still busy.