I use 3M Velcro, and attach as much as the bottom needs for safe attachment, and as neatly as I can manage it; I've put my board upside down and the wah and volume ped didn't move a bit. I take good care of my pedals, and I found Velcro to be the most convenient and least invasive among the methods mentioned; attaching the Velcro via the pedal screws can present problems such as stripped screws, spaces between bottom lid and casing of the stomp that dust could get into, and having not enough Velcro surface for secure attachment. I've never found the need to remove the Velcro, since all my boards use it; the few times I decided to sell pedals with Velcro on them, the buyers actually appreciated that they didn't need to put some themselves, and that it was of good quality.
