Look at what's being used in the field.  For police duty carry, the SLS belt holsters are probably the most popular ones out there.  They're fast, and they're secure.  The drop-leg versions of the SLS holster are quite popular with specialty units, both in law-enforcement and the military.  For those applications they offer the additional advantage that they can be easily painted to blend in with BDUs, or even purchased in several of the more popular patterns.  If you're going to be doing a lot of transitioning from your primary to your sidearm, that's what I'd reccomend.  You pay a lot for them, but you get a fine piece of equipment for the money.
If you don't like thigh rigs, or don't anticipate using your sidearm that often, a double .223 mag pouch can be converted into a holster for a large-frame automatic without too much trouble.  The mag pouch under the weak-side arm on most six-pocket tac vests (the ones with six double .223 pouches across the front) is a good candidate for this mod, since it's also the pocket that it's hardest to get to with your weak-side arm.  Blackhawk even makes a combination pouch with a thumb snap under the cover flap for modular tac vests.  I had one a while back, and it worked pretty well.

-Chris

Deets?  

-Chris

Heh.  It all comes down to making sure of your target, and knowing who is sharing your AO.  Might be worth considering something like that for a mil-sim scenario.  It also comes into play at night.

If Miller's still has some, I'll probably pick up a set of True-Spec ACUs at Miller's tomorrow.  They're not the real thing, but they'll let me evaluate the pattern and features.

-Chris

Haven't had anyone try and grab my rifle, but I did have to barrel-thump some goon not too long ago.  It was kind of half-a**ed, because my sling was looped around my back, but it did the job.  I've tried the same move with my airsoft gun, and despite the bungee inside the high-speed Mamba sling I have on it, I only got about three to four inches of travel.  I'll be getting a single-point sling when I convert my current airsoft M4 to a short-barrel CQB gun, though.

-Chris

True.  And not just for climbing cliffs, either.  Fences, for example, make a sling useful, unless you pass the weapon to a buddy or leave it on the ground.  So do steep hillsides - going in both directions - and climbing up/into structures.
That said, I generally partly unsling my weapon when I go into action.  You can point and shift the weapon better in order to accomodate unconventional shooting positions or obstacles.  And it's also good practise for Real Life.  You won't find anyone - one hopes - grabbing your gun muzzle and yanking on it hard during an airsoft CQB evolution, but it's a viable and effective tactic nonetheless.  If your sling is looped around your body and can't be gotten rid of fast, you're likely to become suddenly dead as you are dragged forward by your weapon onto whatever your opponent has waiting for you.  If you can let go, then the odds are better that you will still have your distance and your balance, he will have your weapon by the wrong end, and before he can correct that you can return the intended mischief with your sidearm.  Also, despite the bungees in many modern slings, barrel-thumping some guy is a lot easier if the weapon is more or less free.  Trust me :-)

-Chris

*wince*  Damn...I thought I'd buried that memory.  My roommate, back when I was ski-bumming, had a penchant for really, REALLY bad movies.  That was one he tried to get me to watch.  The other was Tank Girl.  *shudder*

-Chris

P.S.:  'Cap, at least he's consitent.

Thanks, Greg.  I was thinking the same thing.  Unless y'all are playing in southern New Mexico or around Ajo, there just aren't that many pure desert (i.e.: sand and a few rocks) environments out here.  Everything around here is various shades of medium-to-dark brown with scraggly off-green bushes thrown in, plus the occasional cactus.  A dark-brown based camo scheme plus some faded OD would work very well.  Now, how to fade my IDF vest...hmmmmm.

-Chris

I bought my LBV in OD (ohhhh...so many acronyms, this *must* be for real :-) for two reasons.  One, because I plan on using it both for woodland and desert environments.  Two, because at the time I couldn't find the IDF vest in brown/tan.  As for dulling/painting the nylon, the "shine" of new nylon is one reason.  The other is to break up the hard outline of the vest and help it blend in.

-Chris