There is no "the best" way; there are ways that work for you and ways that don't. Having said that, I posted an article about creating CSS+Image based, fluid round corner technique here:
Box with Round Corners Using CSS and Images - Part 2
An overview of this trick is that that uses nested DIVs and background image repetition and positioning. For fixed width layouts (fixed width stretchable height), you'll need three DIVs and three images. For a fluid width layout (stretchable width and height) you'll need nine DIVs and nine images. Some might consider it too complicated but IMHO its the neatest solution ever. No hacks, no JavaScript.
