First, let's talk proper terminology. When you make a piercing bigger, you are stretching it or enlarging it. When you measure something, you're gauging it. Gauging is not enlarging. Moving on.
The trick with stretching/enlarging piercings is to take things slow and easy.
Understand that your piercings enlarge naturally. Your body doesn't like things to be tight, so it makes space around the jewelry you're wearing. You simply take advantage of that extra room by inserting a slightly larger piece of jewelry, and let the process repeat itself.
Our approach to enlarging a fully healed piercing:
You should notice that your piercing will be more easily able to fit larger pieces if you work on softening the collagen and elastin bonds in the skin around the piercing. This can be accomplished by massaging your piercing with a skin-friendly oil rich in vitamins such as cocoa butter, extra virgin olive oil, jojoba oil, etc. An easy routine is to take out your jewelry prior to bathing, wash the piercing, dry it, then massage with your oil of choice before reinserting your jewelry. You are likely to find that the skin is smoother and more elastic after a few days of this.
Tapered jewelry has also worked well for clients, especially if they find it hard to for their piercings to loosen a whole gauge step on their own. Jewelry such as claw or talon shaped pieces work well. Some allow you to take two gauge steps at a time over a week or two of gentle, natural displacement. The idea is to place the jewelry in as far as it wants to go without feeling strain or pressure in the piercing. You can then place an elastic band/o-ring on the jewelry to keep it in place. You will notice that every day, as you put your jewelry back in after bathing, that it will go a little further, eventually going to the thickest part of the piece.
Most people can move up a size in 4-6 weeks at smaller sizes, 6-8 weeks plus as you continue to stretch. NEVER force jewelry in.
What we DO NOT recommend:
We highly suggest that you do not tear your piercings by lubricating pointy jewelry, a taper, etc. and forcing it through your piercing. The stretch marks and excess scar tissue make the results unattractive compared to enlarging the piercing naturally, and can make it harder for you to enlarge the piercing in the future.