Buy a car with the potential. You don't want to buy a brand new car that doesn't need a paint job, look for a great running car that needs little motor work and a lot of paint and body work. This will make the repairs easier and the repairs will add a higher return on investment.
Test drive. The car you buy has to run good because fixing the motor doesn't really add value to the car in terms of the way it looks. Looks sell in this business and the better your interior and paint look with a small investment, the more money you make.
Time. The amount of time you spend flipping each car has to be low because you want to push out as many cars as possible to sell. So buying cars that need something other than paint, interior or small problems should be avoided. Buying a car that needs a new motor is going to take a long time and a lot of money with less expected value than paint or interior.
Do it yourself. When you first begin to flip cars, you may not have the extra cash to pay a professional paint and body shop to fix your car, but later when you get more capital, your time will become more important. Right now money is the most important thing. You should be fixing the paint, interior and usually the exhaust. Waxing the car and polishing the interior adds value in a short period of time.
Universal appeal. Your car needs to sell in a broad group of people, so using white interior might look attractive to you, the broader audience has to be happy with the car. So using colors like black and red on the outside of the car is ideal. Using lime green paint isn't ideal if you want to sell the car.
Advertisement. Craigslist is your best option because it reaches thousands of people for free. You will need to get clear pictures of the car that show off its beauty.
Negotiation. Depending on how good you negotiate, you can make 5% more or less on each car you flip. Don't act desperate when you sell the car and don't be so easily swayed to go down on the price.