I always work my way up the difficulty of a map until I just can't pass it at all. And then, try Half Time on the maps that are impossible for me (pretty much only Hard+ / insanes now).
Posture is incredibly important, +1.
I can't count how many times I realized I was slouching and raging at how I was missing beats, then SS'd the song after sitting back up.
Wrist support really works wonders as well. I find that having the mouse at the edge of the table, where the flat of your wrist can lie on the edge (works great with rounded edges) allows you enough freedom to move the mouse while keeping your arm from cramping. Of course, the higher the sensitivity of your mouse, the more effective this is - if your sens is too low, you'll need to move the mouse off the table to move your cursor far enough. That'd be a problem.
And I severely disagree with the S-ranking not being challenging point.
More often than not, getting a perfect S on a lower level map is just as difficult as barely-passing a much harder map. (like, say, Sing a 4-star compared to B-ing a 5star)
If you're constantly getting Cs and Ds, you're obviously not doing something right. "Challenging yourself" is no excuse for beating yourself into a ditch. If you enjoy constantly almost-failing, by all means. If you would rather enjoy succeeding and doing well while also knowing you can pull off Cs and Ds on harder maps (no mods), by all means.
I think I'm going to have a severe case of carpal tunnel in my left hand before I get much better. 
