WARNINGS and GUIDELINES: This is more of an art than a science and may require pracice for some. Up-tucks are best done when pretending to tie one's shoes or something of that nature; however,if a boner occurs while sitting down amongst friends, do the following: pretend you are trying to get more comfortable in the chair, your shirt is "bothering" you so you pull it around to make it feel better, while your hands are in the front do the uptuck (underneath the shirt!) then quickly move your hands to the back of your pants to pull them up a little ("they were sagging down too much").
Always make sure you are wearing a shirt or at least boxers. If you have no boxers but do have a shirt, make sure the shirt long and is never lifted up above the waist of your paints.