The trigger pin on my lower (the one that holds the trigger mechanism in place, just forward of the safety selector externally), is a bit loose. I feel it moving a bit when I push it with my fingers on either side of the lower. It came loose once with one side protruding and another stuck inside the receiver, and I had to reassemble the lower to get it back into position. I know that with enough firing, the same thing will eventually happen again. It begins to protrude after running a mag through the gun at the range.

So how do I fix this permanently? I thought about fixing it in place with some adhesive, but I want to be able to disassemble the lower if necessary in the future. Is the hole for the pin too wide, which would mean a problem with the lower itself (it's under warranty)? Is putting in a new pin likely to fix the problem?

1)Ensure your hammer spring is installed correctly. The legs of that spring rest in grooves on either side of the trigger pin, retaining it in the receiver.
See picture from Dano523 in this post:
http://www.ar15.com/forums/topic.html?b=3&f=66&t=509851

2)If the hammer spring is good, then look at the trigger itself. Are the sides of the trigger blocking the trigger pin grooves? If so, lightly stone each side to allow the spring legs room enough to sit on the pin.

3)Is the trigger pin in spec,,,,,are the grooves cut enough to allow the thing to be retained? Get a new pin.

4)Another solution is to use a KNS anti-walk add on to your receiver.
http://www.bravocompanyusa.com/KNS-Precision-Non-Rotating-Trigger-Hammer-Pins-p/kns%20trigger%20hammer%20154%20pins.htm

Thanks a bunch. I'll give all of these a try.

The J-spring in the hammer is missing or damaged.

The J-spring retains the hammer pin.  I think he is describing the trigger pin.  I would check the above post regarding the legs of the hammer spring, if this is not the issue, KNS pins are a inexpensive addition that will prevent both pin walk and pin rotation, therefore wear of your lower.  They are also considerably stronger than standard pins of which I have broken a few from use in aging gun.

My bad!!!!!!!!  Misread OP

+1 on the spring placement... if all else fails buy a set of KNS pins

The Spikes LPKs I just installed all had separate trigger and hammer pins. Never seen it before, but they do exist.





 

I have successfully used loctite to prevent pins from walking/spinning.  You just need to be a little more careful that you don't loctite the hammer pin to the hammer instead of the receiver.  If you ever need to remove it shouldn't take too much to loosen up.  I had the loctite blue.  Not as strong as the red.