I have an A4gery that had a barrel nut come loose, and I was wondering how to avoid this in the future.

Well, yes, tighten it properly is one way :-)

The interesting thing is that it can't come loose -- it has the gas tube through it which would stop it turning and "coming loose".

What must have happened is that the barrel was not fully seated into the upper, and that vibration and heat cycles have freed it up somewhat.

So this time, tighten more than you would normally, carefully watching to ensure that the barrel is well and truely seated. Then back off the  nut and re-tighten to a "normal" torque.

I did the torque, loosen, 3 times bit the first time.  It was good for about 3k rounds, but the last time at the range i noteced I could flex the FSB in my hand.  When I went to retighten it, it ended up in the same spot, but when I went for the next notch, I was going to need about 120 FtLbs.  It was about a quarter there at max, so I just retorqued it to the first spot.  I was using a DPMS wrench and a Snap-On dial torque wrench both times.  I just don't want it to happen again!

Run the face of the upper barrel socket down a lapping plate about three/fout times (400 grit to surface the socket face).  This will give you the needed room to take the nut spine over to the next position .  Also, make sure to not only lube the back edge of the barrel extension, but also the inner  face of the barrel nut.

Quoted: I did the torque, loosen, 3 times bit the first time.  It was good for about 3k rounds, but the last time at the range i noteced I could flex the FSB in my hand.  When I went to retighten it, it ended up in the same spot, but when I went for the next notch, I was going to need about 120 FtLbs.  It was about a quarter there at max, so I just retorqued it to the first spot.  I was using a DPMS wrench and a Snap-On dial torque wrench both times.  I just don't want it to happen again!

If you tightened it down to where it was before, it will still be loose. Put some lube on the barrel nut and receiver threads and let it sit overnight. Then try again to see if you can get to the next notch.