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.

This cancellation will occur when the coils are combined in series or in parallel, but I should quickly note that humbuckers are almost always wired in series (that's what gives them their "heavy" sound).  On the other hand, when you select two pickups on almost any Strat, the coils are combined in parallel which gives more of the "glassy" sound.  (However, I do have a mod that will give you either sound on a Strat.  Read about it here.)

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.

You know those little pickup height adjustment screws?  Well, if you physically lower the middle pickup into the body of the guitar, it will not "hear" the strings quite as well. Consequently, when you combine the two pickups, the sound will be closer to that of the neck or bridge pickup by itself (depending on whether you are in position 2 or 4, naturally).  However, you will now have a "tone generator" producing the 60 Hz signal to cancel out the other coil.  Note that this is irrespective of the position of the middle pickup.  All that matters is that it is wired in appropriately (which it certainly should be already).

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.

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.

For a little more information about this, check out www.blueshawk.info, a site devoted to Gibson's Blueshawk guitar, which includes this mod.  (The Fender Elite Stratocaster in the early '80s did as well.)  The site features a lot of good info besides.