A lot of this depends on the thickness of the blade, and the hair it's cutting. In my experience, regardless of the tactics I use, I notice a significant difference in blade sharpness immediately after the first use if I've grow out my facial hair a bit. This is pretty consistent with the guys I know with thicker hair who use the same/similar cartridges. For finer hair, I'm sure the techniques Dev mentioned will work to increase blade lifespan, prevent rusting, etc.
It's very similar to having a set of professional chef's knives – it's MUCH easier to notice when they start to go dull from being freshly sharpened (0-25% dull), than to measure the loss in sharpness from then on (26-100% dull). In fact, someone unfamiliar with the knives may even think they still "cut just fine", without realizing how much arm strength they are putting into the cut, or how ragged the cut ends of their meats/veggies/etc are, compared with a fresh knife.
The real issue is that there are few effective means to resharpen newer disposables. I recall there being a product on the market that worked really well with dual-blade cartridges, but I haven't seen anything that works with 3-4 blades (which is what I use now).
deano.com 
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