Cleanliness. When making any cosmetic product, you should keep all your tools, glassware, and containers extremely clean. You can sterilize them in a 10% solution of bleach or boil them in water for a couple minutes, then dry completely with a clean towel. You should also wash your hands thoroughly and be careful about what you breathe on. My setup for my person is latex-free latex gloves, a painter’s safety mask, and a hairnet—my hair is short, but you can’t be too careful.
Preservatives, anhydrous. An anhydrous product, i.e. something with zero water or water-solubles in it (water, tea, hydrosol, floral water, witch hazel, most things called ‘extracts’, glycerin, honey, among others), may have its shelf life extended by the addition of Vitamin E in the form of tocopherol or tocopheryl acetate. It should be added at concentrations of 0.5% to 5% of the total, unpreserved product. This isn’t technically a preservative, but an antioxidant–it helps keep the lipids from oxidizing quickly and going rancid. This won’t keep the product from spoiling eventually, but it will definitely help. A spoiled anhydrous product has a characteristic odor similar to crayons. A product with this smell should be thrown away.
Preservatives, hydrous. Any product containing a water or water-soluble ingredient should be preserved appropriately, or refrigerated and used within a few days, and the remainder thrown away. Certain deadly critters can grow in unpreserved cosmetic formulations. If you do wish to preserve a product, and I recommend you do if you make anything more than a small amount or don’t want to refrigerate, you’ll have to do some reading. Preservatives are an annoyingly complex topic. Susan Barclay at Point of Interest extensively discusses preservatives for lotions and other beauty products for an audience of hobbyists. Complete preservative systems, premade and intended for homemade lotions, can be purchased online.