Here is another alternative.  First let me say that from a pure a/v point-of-view,what was mentioned by Simon is the best and right way to do things.  However, in the past I have been dictated by owners and/or interior designers that it must go in the cabinet.  The solution for this which works rather well is the following:
First, to measure the footprint of the subwoofer, lets say it’s 15"wide x 15” deep, then to cut a hole an inch bigger on all sides in the bottom shelf (hole will be 17"x17”).  This will allow the subwoofer to sit on the ‘floor’ of the room, so when a Low Frequency Effect (LFE) occurs, the sub won’t rattle the cabinet, and the nick-nacks that are in it.  If this is a new cabinet being built have the cabinet maker place some cleats where the hole cut is made and a finger pull in the cut piece so it can be reinstalled later if needed.  (By the way, when measuring, make sure that you clear the power cord, and the back of the amplifiers cooling fins, also save this panel so you can reinstall it, should you not want a subwoofer there anymore.)  Also once the floor of the cabinet’s bay is cut you could place the subwoofer on a hard piece of foam to raise it.  The main point of this, is to isolate the subwoofers vibrations from the cabinet.
Second, if your cabinet has a toe kick, you will want to cut a couple of slots in the very front of the bottom shelf so the air(sound) can ‘vent’ from the subwoofer into the room.  As an example if your cabinet’s subwoofer bay is 24” wide (inside finish dimensions) then I would cut (2) 1” wide x 10” slots.  If this is done correctly, you should be able to face the cabinet, and while squating down, place your hand palm up just in front of the toe kick, and your fingers would come up into the subwoofers cabinet bay space, just behind the closed door.  This would allow you to keep all the front doors looking the same and not have some panels that are solid wood, and one that has speaker fabric for the subwoofer.
Couple of things to consider.  If your subwoofer’s is a bottom-firing unit, then you will want to place the sub on it’s side (and measure the cut hole for it lying down) so the driver faces the front of the cabinet.  You can also calculate or increase the amount of volume coming into the room by cutting holes in the vertical divider support that divides the subwoofer bay from the adjacent bay, and then cut an additional set of slots as mentioned above, to increase the amount of venting into the room.

SoundSense has performed several acoustic studies on this problem.  Our original study was on speakers recessed in a wall or ceiling.  We have specifically designed a material called QB speaker and QB subwoofer to prevent some of the standard acoustic problems and anomalies associated with placing a speaker or subwoofer in an enclosure.  One problem with the subwoofer is that the acoustic wave interacts with the structure in which it is placed.  This in turn interacts with and complicates and corrupts the wave from the subwoofer.  QB subwoofer is designed to minimize this problem, as well as a few other problems associated with the subwoofer in an enclosure.  Further questions can be answered by emailing me or calling 877-NoiseOut

Put down the keyboard. Pick up a dictionary.

“If the sub has a reflex port, you’ll need to attach a tight-fitting pipe that leads outside the enclosure. This will probably affect the sound of the sub too. “

“This will have a drastic change in the sub’s tuning. I would never suggest this. You may damage your driver by extending the port because it will lower the tune, possibly well below the driver’s FS. “

Not if you tight-washed the inside of the bass-blower.  Holding the upper half to the bottom, you could easily torch the operator.  The wind-driven cone could be turned all over and revised with turns and dips.

“If the sub has a reflex port, you’ll need to attach a tight-fitting pipe that leads outside the enclosure. This will probably affect the sound of the sub too. “

This will have a drastic change in the sub’s tuning. I would never suggest this. You may damage your driver by extending the port because it will lower the tune, possibly well below the driver’s FS.