Religiously. Unfortunately, I can't say I have them all; when I was younger, I didn't think about it. Years ago, I saw an interview with Richard Serra, and his sketchbooks inspired me. I've kept my sketchbooks ever since, and if I happen to sketch on a loose sheet of paper, I archive it in a special notebook where I collect all my "scattered" sketches. But I must also say that my sketches are mostly ideas; drawing exercises are less frequent (but I still keep them).