|inverted nipples (grades I, II, and in most cases III as well) can be pierced, which will semi permanently evert the nipple.
First, find a piercer who is professional, knowledgable, and clean. Preferably someone with experience in piercing inverted nipples.
Second, determine which grade of inversion you suffer from (as this is not life threatening, I apologize for using the word 'suffer', but it is an abnormality, painful or not);
Grade I inversion means that with a little stimulation/arousal or exposure to extreme or sometimes even slight cold, the nub of the nipple will become erect and evert itself for a short period of time;
Grade II inversion means that the nub can be pulled out, and in some cases can become erect and evert, but only under extreme temperature conditions, and will quickly invert itself again;
Grade III is a little tougher to pierce because, though just about any inversion of the nipple can be everted, it takes alot of force, and will immediately invert itself. Sometimes pulling it out can be painful enough to convince the person not to get pierced.
Now, once an inverted nipple is everted through the force of a piercing, normally and advisably a straight barbell, it will naturally pull a little (depending on the grade of inversion) trying to invert itself again. This normally isn't the problem for Grade I. Keep in mind that nipple piercings are notorious for getting infected, and catching on things, inverted or not. Your main concerns should be;
Cleaning it: Sea salt soak every day for the first 3-4 weeks. Spray with bactine daily. Also, antibacterial soap can work, as long as you are sure you can get it all out of the tunnel.
Correct placement: the piercing should be at the base of the nub of your nipple, to avoid migration. Migration occurs when part of your body pulls the opposite way the piercing is pulling, and eventually works it out of the skin, leaving an unsightly scar, a keyloid (ball of scar tissue trapped in the once pierced area) and a whole new set of problems.
Absess: If the barbell is too short and the nub inverts itself along with the barbell, the skin will grow around it and create an absess that will eventually need to be surgically removed. I personally recommend nipple shields as they hold the barbell out and give enough support around the aeriole and sometimes onto the breast, depending on the size of the aeriole and of the shield. This re-enforcment ensure that the barbell cannot be pulled back. (Tip: Make sure the shield isn't pulling so far out that it will cause migration, discuss this with your piercer)
Once an inverted nipple is everted through piercing, it will build scar tissue and may eventually stay everted (permanently in some cases) even without the jewelry in. However, don't take the jewelry out too early, or else it may heal over, invert itself and need to be done again. Depending on the amount of scar tissue buildup in the nipple, it can usually be done again.
Talk with you piercer and make sure to ask his or her expert opinion as well. I hope this was helpful.