Contracts and liability waivers. Cover your ass because people will blame you for putting on the dark tint they asked for and then got into an accident with because they couldn't see.
Get a down payment, at least 50% for automotive. You'd might be surprised at the amount of people who want their windows tinted, call and set up an appointment, then never show up. Leaving you sitting and waiting for nothing.
Be very weary of low cost films. Some are actually decent, but most are either hard to work with (won't shrink well) or won't last long. Even if the adhesive comes with a good warranty it doesn't really matter if your 20% vlt looks more like 70% after a year because the dye faded. A dyed polyester film will hold it's vlt and color much better than a dyed adhesive.
Also, don't use automotive film on commercial windows. Most auto film is not double pane compatible and may cause double pane windows to break. If you have double pane glass, use a film specified for double pane.
If you're doing auto, learn the law in the area you'll be tinting. It's becoming more common for the police to cite the installer of dark vlt's instead of the owner of the car, or both. Be prepared to tell people you don't install illegal VLT's and have then argue with you or try to get you to anyway.
You may also want to install protection film on your shop windows, since pissed off 16 year olds who aren't happy you didn't put the 5% vlt on for them or tint their windshield may throw large rocks at your windows.
Most of all, do only what you're comfortable with and use common sense.
Otherwise - with auto - once you learn to dry shrink a backglass without messing the film up and get a system down, you could tint a 4 door car in about an hour (it takes me about 2). Depending on the area, you could charge $250-$400 for that. Plus, upgrades on film types are a good way to put extra money in your pocket. As well as "artistic" upgrades like cutting flames into the film, etc etc.
There's always a handful of low ballers using the cheapest film they can find and charging $60-100 for a 4dr. You'll be redoing their work, so you should also learn how to remove the cheapest crappiest film on the planet.