If it smells, or if there are noticeable stains, or I've been sweaty, I wash it. But if not, I don't. None of my clothes are really expensive, but usually I just use cold water and hang dry. Sometimes I'll put sweaters in a bucket if I'm worried about them shrinking. But I'm not using a washboard, or anything. If I'm confident my shirts won't shrink, I'll use warm or hot water. In general, shirts need washing more often (well, not including socks and underwear).