I tend to buy three different sizes of boxers and boxer-briefs (usually XL, 2XL, and 3XL), often in identical styles -- for the exact reason you cite in this article: fluctuating weight in the future.

Most of my shirts tend to fit moderately well, despite my fluctuations, as I buy half XL and half 2XL. I tend to wear sweatpants when I go out, and pajama bottoms when I'm around the house...so I have less of a problem with pants than most people do.