Personally I have been following the forearm method [1] of keeping my disposable five-blade cartridges sharp. Maybe it won't work for everyone, but it certainly seemed to make a difference to me. Then again, I also grew out a beard so now I only shave a part of my face, and generally do so twice a week, not every day. Having said that, my most recent cartridge lasted me just about a year.
[1] http://lifehacker.com/5502841/extend-the-life-of-your-razor-...