I just came across this video by Crochet with Tuula Maaria for a fascinating technique: How to triple any yarn as you go: The continuous looping method.
I bet this would also work for knitters.
Most of my stash is worsted weight stuff, but the cool thing about this technique is that it will work with whatever yarn you have, if you have a project where you need the yarn to be bulkier.
Check it out!