The key is to have a good sling-back chair and to keep both feet squarely buried in the sand. I find having a low table on your left-hand side helps keep the sand from sticking to the base of your beer bottle, but you can always use a paperback for that too.

In the end, I find it's more about shade than ergonomics, especially in tropical climates where you're dealing primarily with palm trees and their small cross-section. I guess your mileage may vary.