I did it.  I bought a tactical sling for my Colt M-4 today at a gunshow in Portland.  The devil made be do it.  It looked great on the model.  I brought it home and managed to get it installed correctly.  Now how in the hell do you adjust the stupid thing?  I have an advanced degree in Engineering and I can't figure the thing out.  Seems like there are too many loops.  Any advice for a dumb ass?

What kind of sling did you buy?

There are some good pictures on Blakhawk's website.  Follow the link and click on "Product Catalog", then "Slings", then "Slings, Tactical Rifle" and finally "SWIFT Tactical Sling":

[url]http://store1.blackhawkindustries.com[/url]

I think this is a fairly typical tac sling design.

If you look at the picture at the top, you'll see a bar-slide on the sling just behind the guy's shoulder.  Remove the free end from the bar slide, then pull on the free end to make the loop smaller (or vice-versa to make larger) move the bar slide to the appropriate  position on the sling and re-attach as before.  

Sorry for the long-winded reply.  I guess a picture truely is worth a thousand words (or at least a couple hundred in this case). :-)