|What's a humbucker?
Humbuckers are almost always pickups with two coils, like those typically found in Les Paul types of guitars. They get their name from the fact that they eliminate the obnoxious 60 Hz hum common to most single-coil pickups (I'll go into detail about the exceptions a little later). This is accomplished by the fact that the each coil is wired the opposite direction. As a result, the 60 Hz signal produced by each coil is out of phase with the other. When they are combined, they cancel one another out.
Mod #1: Go low
Normally you don't have many options if you want to get the great bite of a single coil sound without the hum. A single-coil by itself is going to hum, but when you add in another pickup, you lose the bite. The trick is to remove one aspect of the extra coil but not the other.
Mod #2: Caged coils
An alternative approach that I have never tried is to cover the middle pickup with something like a piece of copper. Basically, you could have a miniature Faraday cage block the magnetic field from the middle pickup. I believe you would have to run this to the ground (electrically speaking, of course), so that might be more trouble than it's worth.
Mod #3: Dummy coils
Yet another possibility is to build a so-called "dummy coil." Unfortunately, I am unfamiliar with how to include something that would produce the "counter signal" at the appropriate amplitude. If anyone has that knowledge and would care to enlighten me, please feel free to email me.
The exceptions I mentioned above regarding Strat pickups that are not true single-coils include models like Lace Sensors and those made by EMG. While these have the appearance of single-coil pickups, they actually contain two coils wired similarly to the configuration that "bucks" the hum in conventional humbuckers. However, depending on the model, they do a decent job of emulating the "quack" of standard single-coils... which I happen to like.
If you would rather apply your credit card than a screw driver, I would recommend either the Gold Lace Sensors or EMG's SA pickups. Both companies produce other varieties of "single" coils, but these two provided the sounds Strat lovers are typically looking for.