There are few ways to earn lots of money quickly in this game. Missions often take a lot of time and can be expensive on ammunition/missiles (as well as the skills required to use them). Manufacturing is expensive (at least when I've tried it producing a Caldari shuttle) and can result in a loss because of the high costs of blueprints and raw materials.
However I've found a boring (yet productive) way to riches. If you go to (almost) any stargate you will find cargo containers floating around. These are generally left behind after a ship is destroyed. If you approach these within 1.5km you can open them and transfer what's inside to your cargo hold. Once your cargo hold is full (or you've run out of cargo containers) jump back to the nearest station to sell them on the market there. If you can't sell them, keep them in your cargohold either for fitting to your ship either now or later or to try selling elsewhere. The same tactic should also be used on missions to supplement your income enabling you to progress further in the game more quickly.
Completing missions within the bonus time and accepting your agents offers (which are often for free stuff or high prices for easily obtainable goods).
There are some who say that looting cargo containers is unethical as the owners might want to come back within the hour and claim their goods. However in my experience the vast majority of cargo containers orbiting stargates are from NPC (non-playing characters) who wouldn't go back to collect their items. The best thing I ever found in this way was a 1.4 million ISK item. However this was in a lower security system (think it was 0.6 space) where the chances of somebody gate camping and using this as bait in a trap in higher. Having said that though IMHO the rewards outweigh the risk. The process is also much speeded up with the use of an afterburner (and the tractor beam if you can afford one - although I've never bought one myself).
