My last two workouts I changed the way I do dips and pullups.
My lower body gets so relaxed sometimes while doing dips and pullups that I have strained my lower back a couple of times just from the extreme stretch.
I started pulling my knees into my chest(as best as I can) while doing them and they are pretty tuff. It's weird focusing on those two actions at once, and I can feel a cool abdominal contraction in near what I think is my diaphram. Anyway, thought I'd share. Later!
I usually either cross my legs behind me or bring them up to my chest like you describe.
-doug-
Try doing really slow dips, works good for pulls too. Try doing 30 seconds, or a minute. It's really hard to go that slow at some parts of the rep.