As a spray tanner who has been doing it for over 7 years I hope I can help.
Exfoliate thoroughly for about 3 days before your tan, paying special attention to your elbows, knees and ankles. The reason for this, is that these areas can get particularly dry and have a tendency to absorb more tan - hence the reason why the tan stays longer than on any other part of the body.
Moisturise your skin well for about a week before you plan to tan - it'll help it to last as long as possible. Don't shave or moisturise 24 hours before your tan. Some razors have a lubricating strip which puts a protective layer on the skin, forming a barrier against the tan working. Try to avoid using soaps and shower gels that contain moisturisers 24 hours before, for the same reason.
Don't wear deodorant, perfume or make-up to your tan session. Take some babywipes with you if you've had to wear deodorant or make-up befoer your session. Any deodorant will temporarily turn your armpit area green - don't worry - it will come off the next day.
You will look quite brown after your tan - this is the "guide tan" - think of it as foundation. This will come off when you shower the next day. Don't panic if you look darker than you want to be.
Leave your tan on for at least 8 hours - overnight is better - do NOT shower or bath before the tan has time to develop. Only "rapid" tans can be washed off after 1 - 3 hours.
Depending on the brand, tans can come in 6, 7.5, 8, 9.5, 10, 12, 14, 16, 18 and 20%. You can also mix some brands so that a customised shade can be created for the client - mine do that. Your tan tech should be able to advise what is best for your skin type. One only goes orange when too high a percentage is applied ie the tan is too dark for your skin-type 
If you already have a base tan, then hopefully your tan tech will tailor it for you after having done a full consultation.
Wear loose, dark clothing - tight leggings and tops will take your tan off before its had a chance to settle on your skin.
Some tans are what they are the next day, some will continue to develop over a 48 ours period. I won't go into the details but some tans contain ingredients which mean you reach your optimum colour 2 days after your tan. If I have clients entering dance comps, I always tan them two days before, if I'm using certain brands. If I'm tanning brides, I ensure I tan two days before with a tan that reaches its optimum the day after, as some people find some colour still comes off on their clothing - that's something you'd want to avoid with white or ivory fabrics. If it's important to you, ask your tan tech when their brand reaches its optimum/darkest.
As someone who specialises in dance/performance tans, I offer a menu of body areas that can be tanned but not everyone does this. if you don't need a whole body tan, then ask your tan tech if they can spray specific areas.
If you need more tips then let me know