I have been using Norton 3X ProSand sandpaper for several years now and I am super impressed. I get it through Lee Valley Tools when it goes on sale because normally it ain't cheap! This stuff is supposed to last 3x longer than others, but when I compare it to the stuff I get at the local hardware store I would have to say it lasts 5 times longer or better!
It just went on sale again at Lee Valley Tools in a set of 80 sheets, 20 sheets each of 80, 120, 180, and 220 grit for under $40. The kicker is that if you order more than $30 you get free shipping during this sale.
Trust me on this, you will NOT regret this splurge. I use most of my sandpaper on a quarter sheet palm sander, and a quarter sheet will hold up for three or four bows! I put my sandpaper sheets in one of those expanding file folders with a pocket for each grit. When I switch down to a finer grit, I just pop the used quarter sheet back in the appropriate pocket for the next time I need it.