What is your skin tone now? If you have fair skin, you'll want to choose a sunless tanner that offers a light shade, or else promises a gradual tan. If you have darker skin, you may only see noticeable results from a medium/dark shade of tanner. Tanners that only offer one shade tend to be pretty dark and intense on light skin, but light and subtle on dark skin.
What's your skin type? If your skin is dry, a lotion-tanner is best a best bet, whereas mousses and gels are better for normal to oily skin, according to SheKnows.com. Wipes can work well for most skin types.
Have you ever applied sunless tanner before? Lotion formulas are easiest to apply, but they take awhile to dry. Gel or mousse formulas dry quickly, but they take a more experienced hand (or, at least, more caution to avoid streaking).
How long do you have? Even instant tanners take a few hours to develop, but gradual tanners can take days to see results. One quick shortcut is tinted tanners, which can give the impression of darker skin immediately -- though some tints tend to look splotchy or fake before they're washed off.
Is scent important to you? Most self-tanners have a chemical smell, but some are more tolerable than others. Take a quick sniff test before buying.