The obvious answer is 'chuck out more C02'
This is an insanely dangerous thing to do. The issue with C02 as a global warming gas is not how much heat it traps, it's how long it sticks around for and the fact that it's much harder to put 'back in the box' than it is to let it out. The carbon cycle is a very slow moving process when you look at the average atmospheric lifetimes of various gases. I'm not going to put down any numbers here, because they vary wildly depending on which research paper you get them from and the starting assumptions used, but it's safe to say that if you're pumping out C02 you aren't really going to feel the effects, but your great grandchildren certainly will.
As a result: Using C02 to rapidly raise the earth's temperature is a very bad idea. A C02 output that raises the temperature by 1 degree quickly is going to have a lot of very long term consequences that you can't predict (and would almost certainly be Bad). It's also worth pointing out that some of the sulphuric compounds released by burning fossil fuels can cause global cooling over short periods of time, but they leave the atmosphere much faster. If you increase your coal burning fast enough, however, you're going to drop the temperature first!
As Bowlturner has already noted, Methane is also a powerful greenhouse gas, with a much shorter lifespan than C02. Production of methane is also relatively simple: get biomass and let it decompose in an anaerobic situation. Marshes and creeks do this naturally already, but you can quickly get a public awareness campaign together to get everyone doing this 'for the good of the world'.
Another powerful (but often overlooked) greenhouse gas is water. If you couple large scale nuclear power generation (cooled by water towers or by having a heat exchanger under a lake) with distributed water heaters (powered by the power stations) you can literally pump heat into the air at the same time as raising the global humidity. I have no idea what havoc that might wreak on the climate, it would take a long time to build, and it's really not very efficient given that you've got to constantly pump in energy, but it might help raise the temperature in a way that won't cook the planet once you're done. Plus I like the idea of going for a swim in a nuclear powered lake.