I'll echo Travis' thoughts here. The first things are patience and knowing your subject.
Older manual focus lenses are the way to go. For birds it's better to have a 600/4 manual focus lens than the latest 300/5.6 with image stabilization and autofocus, which will still cost more than the manual 600/4.
1.4x converters are usually more tolerable than 2x converters.
The tripod will be more important to sharpness than the lens in most cases. Most complaints one hears about someone's new super-tele have more to do with not having an adequate tripod and head and technique to use them than with the optical limitations of the lens.
Shoot a lot of film. Even if you think you got it, there are all kinds of chance and random factors that can get in the way--subject movement, camera movement, wind, etc. Don't be afraid to toss half of your slides for technical reasons, and half of the ones remaining for aesthetic reasons.