Even the most seasoned sawyer can dread a long session of ripping down stock by hand. There are quite a few ways to prevent yourself from burning out, but my personal favorite is to use different muscle groups as I go along.
I will start by ripping on my saw-bench, kneeling on my work. As I go along I switch to this position.
It may look awkward, but it’s rather comfortable and gives you an excellent view of your saw for keeping it both plumb and on your cut line.
It’s hard to see, but you can actually use your thigh to brace your work at the end of the cut so it comes away cleanly.
A few other things you can do are:
1. Rip and crosscut your components in stages as opposed to all at once. Get those components all finished and ready to join and move to the next stage.
2. Use wax on your saw-plate, I forget to do this often, but I am always amazed by the difference this makes.
3. A tiny wedge like this one can take a lot of pressure off of your saw. Make a few dozen at a time, you will both use them and lose them.
-- Make furniture that lasts as long as the tree - Ryan