This is by far the cheapest, easiest, fastest, most automatic and most feasible method. We could start this today if we wanted to. However, the social, international hatred of nuclear devices will probably prevent this method from being used. A class of primitive bacteria called Methanogens can survive on today's Mars, and would create methane, a greenhouse gas, from carbon dioxide and hydrogen contained in the regolith. This method is also very cheap, easy, and automatic, but it is not very feasible, as the methane would last mere hours before being broken up by the sun's powerful UV rays.
Perfluorocarbons are very powerful, long-lived greenhouse gases.
Fluroform is 10,000 times more effective a greenhouse gas than carbon dioxide, and lasts centuries even in Mars's tenuous atmosphere. It is made by releasing pure fluorine into the Martian atmosphere. The fluorine reacts with carbon dioxide to form fluoroform and oxygen. Automatic factories could be inserted on the martian surface that would mine apatite for fluorine, using solar power. The main advantage of using PFC's is that it is the most feasible method that is socially acceptable.