People might top them up every day (or in my case, once every couple weeks). They don't pump them up from flat every day!! I pump mine to somewhere between 90 and 100 psi ... and a couple weeks later when I check, they're down to maybe 75-80 psi, so I top them back up to somewhere between 90 and 100 psi.
By flat, do you mean flat? As in ... no air? Or do you mean ... ever-so-slightly squishier than the were a couple days ago?
If your tires are going completely flat every day, there's a problem. If I were you I'd very carefully change the tubes and tires.
I dont get a flat everyday, I just mean that I release the pressure and repump them to make sure pressure stays at 120ish. This is what my LBS told me to do, I dont have a leak but what they told me is that at that pressure the tubes will just bleed some air due to the high pressure.
From what I hear from people is that maybe I dont need 120, but from 90 (the pdf) to 110 (recommended by sheldon).