Bloodsong -- pardon me if you've already got this, but it sounds like what you're looking for is the "=" key. Select the poly's you want to work on (or select the entire surface using the stats ("w") window) then hit the "equals" key to make everything else invisible. Operations you perform won't affect the hidden poly's. Be careful when using knife or bandsaw, as this will result in separated edges (the extra point will exist in the sliced poly's edge, but not the adjacent hidden poly's) -- you'll have to weld the new points on the ends to existing corner vertices to get the edges joined again. And hit the backlsash to unhide things.