Something that I've done recently which seems to work well ...

At most LFSes (inc. PetSmart) you can find "Lee's Discard-A-Stone"
airstones. See: http://www.marinedepot.com/a_ap_lees_as.asp

These are discardable airstones that come with separate plastic barbs; if
you were to use an airstone, you would fit one end of the barb into the
airstone and the other end into your tubing. One end has a (what's the
term I need) collar which would normally seal the end of the airstone.
Crude ASCII rendering if you were looking at the barb from the side:

=|==

I drill a small hole into the center of my CO2 bottle's plastic cap, just
small enough so that I can push the barb through (from the cap's outside),
so that the collar is on the outside of the cap:
_
|
=||=
|_

The end of the barb you're pushing through the cap is slightly tapered; try
to make the drilled hole as wide as that end, and then when you push the
barb through, exert some force and you'll get a good seal around the barb.
In addition, the collar against the cap's outside helps to seal the hole.
I put some aquarium sealant around the collar, but it's probably not
necessary.

Previous setups I've used have actually had the CO2 line glued to the cap,
and I'd have to rotate the bottle to unscrew the cap when I wanted to
change the mixture. Not a big deal, but this is even easier; just plug and
unplug the CO2 line from the barb.