A couple things - first, I'm sure someone has linked this previously, but I just found it myself, and think it is the most helpful/coolest customizing tip I've come across. Second, I hope it's ok to link to a tutorial from another site's forum:

http://goodtogocustoms.com/forum/index.php?PHPSESSID=bba04cc04a3484eb77cf28514e5b11d3&topic=3963.0

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I just use the grinding stone that came with my Dremel and push it in 3/4 of the way.

It makes the perfect sized hole every time.

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I really wasn't having any trouble making the right sized hole - for me the issue was that I couldn't get that same seal or "pop" noise when putting the head on or off. And using epoxy wouldn't create that either. But the glue - once it cools, is flexible enough to create a lip which makes that seal, almost as good as the true 25A heads. And this method custom fits the head to whichever neck ball you are using - since some are clearly different sizes

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I was going to start a new thread, then remembered this one was around. Sorry for the resurrection, but figured it best to keep the information all in one thread.
I've been working with this technique for quite some time now, and think I have it perfected, or at least enough to share. I consistently get the tight fit and the 'pop' whenever removing one of my modded heads from a figure.

Equipment you need:
Pin vise
X-acto knife
1/4" drill bit (called 'large bit' in the instructions)
1/8" drill bit (called 'standard bit' in the instructions)
1/8" masonry bit (called 'masonry bit' in the instructions)
Tape (masking tape is recommended, but any tape that leaves little or no adhesive behind will work)

For those not familiar with masonry bits, they are drill bits for drilling into brick, concrete, and other masonry. The tip has a flared cutting surface to it that is slightly wider than the shaft of the bit. Luckily for us (at least with Joes), this flare is approximately the right height and width to match the neck balls of 25th Joes. (Bit sizes needed vary depending on figure-style modding, hence not using the specific sizes in the tutorial. To determine the proper size, hold the tip of the masonry bit to the neck ball. The masonry bit with a flare equal to the neck ball is the bit you need. The standard bit you need will be the one that is the size of the shaft of the masonry bit. The large bit needs to be at least as wide as the neck, but not as wide as the head. The more pliable the material the head is made from, the easier it is to work with this method.)

Step 1: Trim the head to the proportions needed. For my customs, I follow the jaw and neck line, making an angled cut along the jaw line, then a cut from the back of the jaw to the hair line.

Step 2: Determine the depth of the neck. To determine the overall depth, hold the head up to the figure's neck and raise it or lower it until it looks natural. Place a piece of tape on the figure's neck at the point the lower portion of the jaw and the neckline meet.

Step 3: Mark your bits. To mark the large bit, hold it so the point goes toward the body of the figure and place the tip at the top of the tape (say that three times fast). Mark with a piece of tape on the shaft of the bit approximately 1/16" below the base of the neck ball. To mark the standard bit and masonry bit, hold each in the same fashion as the large bit, marking with tape at the top of the neck ball.

Step 4: Drill for depth. Place the large bit to the bottom of the head, centering between the two sides and centered between the back of the head and midpoint of the jaw. Drill into the head until the tape mark is even with the bottom of the jaw. Place the standard bit in the center of the just-drilled hole, and drill until the tape mark is even with the bottom of the jaw.(note: I generally hold the large bit in my hand, as I can turn it easily without the pin vise. I recommend using the pin vise with the standard and masonry bits, though.

Step 5: Bore out for the neck ball. Shove the masonry bit into the hole drilled with the standard bit, taking care to push it straight in and not twist it. We only want to remove plastic where the neck ball will reside. Once the bit is fully in, checked against the tape mark aligning with the bottom of the jaw, begin to rotate the bit, taing care to not apply forward pressure. I generally give ten or so turns clockwise, then ten or so counterclockwise. We don't want to actually drill into the head any more, just widen out the bore where the neck ball will be.

Step 6: Test. Pull the masonry bit out, again avoiding turning it as you remove it. Place the head on your figure and check for range of motion. If the head seems too tight, remove it and give the large bit a few more turns.

Having now successfully done close to twenty heads this way (don't ask how many unsuccessful perfecting it, just glad Dollar Stores carry Chap Mei practice fodder), I can say that it yields a head that fits as well as anything from the factory.

I hope my instructions are clear, but if not, let me know and I can try to do a pictorial version to accompany them.

Trying sooo hard not to make a joke but its killing me.

As to the topic at hand, I find the softer new sculpt style heads work really well with just drilling them out a bit. The softer material makes it easier get a good fit. But for the older ARAH heads and cast heads the glue-gun method works well too.

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Seriously, that is the most disgusting container of vasoline I have ever seen. Its even worse than the one in Kill Bill.

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Between this glue gun trick and the other trick I learned on how to sculpt caps out of epoxy directly on to the figure - while still keeping them removable - is it too weird to say that two of my favorite things involve the use of Vasoline? Not that there's anything wrong with that

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