I get it to work nearly every time, and I have an old trackpad with a button (imagine a trackpad that is slightly smaller than the unibody ones, then lop off another 2/5 for the button).
I have 4 fingers swipe up as mission control, but if I put a thumb and three fingers, and swipe the three fingers up, it activate mission control, not show desktop - thus the key is to actually move your thumb as well.
I think of it just like the 2-finger pinch/spread, except the index finger is three fingers joined as one unit. that is, just do the 2-finger spread/pinch, but lock your middle and ring finger with your index.
Yea, I use a 5 finger swipe "out" to show the desktop, and a 5 finger swipe "in" for launchpad. No need for either one on the doc, and it actually makes launchpad useful with such quick & easy access.