In my experience I have seen many guitarists waste their practice time.  Even though they may know what they should be practicing, they are unsure of how to go about doing it and dont see how this will benefit them.

There is no one practice routine that will suit everybody, everyone has different tastes and time restrictions so the best way is to find your own routine that suits you. The best way to practice is consistently over time. You will benefit much more from picking up your guitar every day for 30 minutes then for 2 hours on a Sunday afternoon.