It's important to familiarize yourself with the types of customers in the game.
There are 12, each with different degrees of tipping and patience. Here are some examples of the customers paying habits and their temperaments.
You can also look at the "How to Play" tab in the game menu:
Great tippers but impatient: Businesswoman, Businessman, Bride (pink dress with tiara)
Okay tips and patient : Woman with the brown skirt, Man with grey pants, Prom queen (little pink dress)
Okay tips and patient : Male student
Okay tips but Impatient : Female student
Bad tips and Impatient : Punk boy and Goth girl
Bad tips but very patient : Grandma and Grandpa
* In the beginning, save up some to get a hair washer and hair dryer (employees) - this will help you a LOT. The coffee is somewhat useful to have (as is a barrista) when you're in a tight spot. Get the most extras you can get as soon as possible - extra hair washing chair, extra waxing station, etc.
* Also, you can replay levels later on. You will have your newer equipment, and you can gain more cash that way. As far as I know, there aren't any cheats. And as soon as you can buy the candles, go for it. They are REALLY helpful in a tight spot (everyone getting impatient and all).