my method doesn't have the stelllar results that Ingrid's does, BUT I haven't had too many problems with it lately.
I estimate 1 stitch per inch of yarn. (i know, this won't work with all yarns, but it's a good gues s for worsted). So if the pattern calls for 30 CO stitches, I "guesstimate" 30 inches of yarn, by pulling out a strand from my nose to my fingertips. then i pull out a few more inches to be safe. Since that length is, on me, almost exactly 30 inches, I round up everything to a multiple of 30... so for 100 stitches, I would pull enough yarn for 120. Again, it's not exact, but since I started doing this a few months ago (after I had to caso on 300 stitches and started over 4 times) I haven't had to redo but a few times, and that's b/c my tail was only 2 inches long, and I wanted more to weave in.