There's not a whole lot you can do to adjust a spray tan once it's applied and developed, because it takes a few hours for the tanning ingredient, DHA, to develop on the skin. That's why, for this one, it's all about preventative dialogue between yourself and the spray tan artist. Use words like "sun-kissed," "golden," or "deep bronze," to describe the shade you want. Tell them, "I'd rather it be too light than too dark," or vice versa. This tells them which side of caution to err on. Finally, bring in a visual reference, whether it's a photo of yourself, a spray tanned friend, or a celebrity. It will ensure both of you are on the same page before the spray tan is, well, sprayed.
Also, if you're getting a spray tan before a big event, like your wedding, go in a few weeks earlier for a test. Describe the result you're looking for, see how the colour develops on your skin, then adjust accordingly the next time you go in. It prevents you from being disappointed with the result.