Flies are annoying and they're difficult to catch, so your levels of frustration can break through the roof if you spend too much time chasing after them to no avail. Fortunately, Wired Magazine found that the answer to your aggravation lies in our good old friend science.
Flies are so hard to swat because they have a 360-degree field of view. This allows them to see you coming, allowing them to begin a series of postural adjustments so they can fly away quickly -- well before your hand (or whatever) deals the crushing blow. Understanding the fly's biology is necessary in the war against it and its will to live. It's faster than you, but it isn't smarter. Wired suggests the following two methods for more effective fly-swatting:
#1: If the crushing blow approaches head-on, the fly shifts its middle legs forward so it can push backward. Anticipate the backward jump by angling the swatter to arc over and then behind the fly.
#2: Perceiving a threat coming from behind, the fly moves its middle legs backward in preparation to launch forward. To land the swat, you have to lead the fly "like a quarterback leading a receiver."
There you have it. You can't beat flies on speed, but you can outsmart them. Use your wits and you should find yourself more successful.