LD14 was a freakin blast! It is my first LD and definitely not my last. 48 hours isn't as short as it sounds... I had time to complete my entry, and had plenty of time to eat/sleep/shower/run errands without feeling particularly stressed. You still have to be pretty one-track minded... fortunately, the subconscious is pretty good at working out problems even if you aren't actively thinking about them.
I think what helped me through the weekend was having a solid idea in my mind of what I wanted. I spent maybe 6 hours after the theme was announced sitting, walking, biking, and thinking of a good concept. This is definitely not wasted time. Without a solid idea you will just spin your wheels and end up frustrated and without something to submit. Also, if you haven't invested in a whiteboard I highly suggest it! Being able to map out all my objectives helped me keep focus, and marking them off the list boosts morale.
One of the LD traditions is to record a timelapse of your weekend. This is a great idea, and a neat way to watch a concept evolve. I particularly like watching how the game grows from one red cube on the screen to the final product. Also, I talk on IM waay too much ;)