So many things to choose from :)
But I'd say the most important thing I do for ALL my sites is apply a little Thought & Consideration.
I try my best to ensure it works in both 800 and 1024 resolutions, that fonts are scalable in all browsers, easily legable and whenever the design permits, make things fluid/elastic using em's and %.
Certain designs are obviously out of the question - I (personally) detest any design that "breaks" within a few text size alterations, or when you reduce/expand the window width/height - I just view that as lazy or ignorant (if not both).
The site in many cases, (though not all), is aimed at being informative... if people cannot read things due to their current setup, they should at least be able to easily alter it so that it is available to them,
(that said, I saw someone test a design at over 1500 Res last week... why?).