The most effective way that I've found is to push your boner to the side, if that makes sense. The problem with this is that you gotta hide the act of pushing your boner to the side.
What's a better way? Don't you dare say folding your arms...
-
[spoiler]Folding your arms.[/spoiler] But seriously, put it up, as in aiming in toward your chest. It makes it to where you can't even see it, but to where sitting down is awkward as hell, with the added consequence of everyone thinking you have nothing down there (which applies to all the wimminz who crave for the D).