For a horse like this, I would agree with KV and use a header while getting on just to keep him a little more under control until you can get centered in the saddle. After you are on, like everyone else said, don't look at the ground. I have found that I have the best luck when I keep my eyes right on their mane just in front of the saddle. Since you are riding western, I greatly recommend a nightlatch for your saddle. They are much easier to hold on to and keep you much more secure than just grabbing the horn.
Also, once you know he's going to buck, if you have time, take one rein and bend his neck. That likely won't stop him but it should take away a lot of his power and keep him from switching directions suddenly on you. On a horse that is known to buck or a very green horse, I spend much of my time riding with one rein shorter than the other so that I don't have to re-adjust in the event of an emergency, I can just pull and it will bend them.
After they are stopped, if I manage to stay with them, I will keep them bent and push them into little tiny circles (very similar to a one-rein stop only I keep pushing them to disengage those hindquarters) as fast as I can get them to go without feeling out of control. I don't give them even a moment to stop and breathe between the fit and the circles unless I am almost falling off and need to re-center. If I begin to get dizzy before I feel like they've had enough, I'll switch reins and start them in the other direction without giving them a moment to rest.
Before I let them out of that circle, I expect them to be moving off my leg quickly (not frantically), and their face/neck should be soft and relaxed/lowered. Then, I take the leg off and keep them bent until their feet stop. Done correctly and often, 5 minutes of those little circles will suck more piss and vinegar out of a horse than loping them 5 miles in a straight line. Normally, a bucker will quickly figure out that bucking = work and they'll stop...normally.
ETA: This is also one method I use for teaching a young horse to spook in place instead of spinning and bolting. It works wonders on some horses.
Last edited by smrobs; 07-11-2011 at 05:02 PM.