My preferred aftercare method is to use a Sterile Isotonic Saline Spray (also sold as “wound wash”) to rinse the healing piercing once or twice a day (up to three times a day, if you got it extra dirty throughout the day).

Definitions:
“Saline” is a mixture of Sodium Chloride (NaCl— or more simply “salt”) and water.
“Sterile” means that there are no living microorganisms (unlike tap water— yes, even clean tap water like the kind we have in this city can still contain life).
“Isotonic” means that it is roughly the same salinity as the human body (9mg of NaCl /mL of water, or 0.9% salinity). This is important because saline that is too salty (hypertonic) will dry out the wound, and if it is not salty enough (hypotonic), it will cause the body to absorb too much water. An isotonic solution keeps the moisture levels right where they need to be.

I carry NeilMed Piercing Aftercare spray.
It is available in a “fine mist” spray, which is more gentle than other options.

It is NOT recommended that you make your own saline solution, as it is really easy to mess it up with catastrophic results. The most common mistakes that I see are folks using way too much salt (violently drying out their piercing) or using the wrong kinds of salt (for example: regular table salt contains iodine and other additives, including sugar, which will irritate the healing wound SO MUCH. Epsom salt [Magnesium Sulphate or “MgSO4”] is an entirely different chemical compound than salt and does not do the same thing).

If you are going to make your own saline (to rinse out your mouth, for example), the safest and most fool-proof method I’ve found is to use pre-measured packets of salt meant for Neti Pots (or other nasal irrigation devices). NeilMed also makes those and they can be found in most pharmacies(but don’t get the extra strength— that one is hypertonic!)
Mix according to instructions: One packet of salt per 240mL of water. If using tap water: boil for at least 5 minutes and allow to cool. You could also use distilled or filtered water. Alternatively (if you are being careful), you can use 1/4 TEAspoon (or about 2 grams) of ADDITIVE-FREE salt per 240mL of water (kosher salt and pickling salt are both iodine-free as a rule, but double-check the ingredients to make sure it’s only salt with no other additives.)

Always make sure you wash your hands THOROUGHLY before handling your piercing. Don’t just rinse them— use soap and warm water and make sure you scrub for at least 20 seconds!