There's a Chinese proverb that goes something like this:
If you wait by the river long enough, you'll see the body of your enemy floating by.
I haven't quite figured out why yet, but this really speaks to me. I think there's a lesson in there for testers. Your enemies are the bugs in the system. If you really want to find them, you may have to live with your system for a while. Not all bugs are immediately apparent, no matter how thorough your testing is. Give those intermittent, or time-based, or volume-based, or subtle bugs a chance to go floating by. Live with a system or a feature for a while; your customers will and so should you.
Sometimes finding the bugs just takes a little patience.