Anyone who tells you how to get rich easily, be afraid, it probably won't be that easy! That goes for EVE too. There are some good ways to increase your stock of ISK, but they all involve effort, investment, strategy and even luck, okay?
My favorite way to make money in the game is through trading. Especially items made available at events (ships, projects or other items), where demand is high and consequently so is the price. So taking advantage of update periods and the start of events in the game turns out to be one of the most profitable ways possible, but as I said before, it requires dedication to farming the items.
When there isn't an attractive event going on, the most lucrative way of making money in the game for me is by exploring for relics in Null-sec. Obviously, this requires a late-game account for survival reasons. Metallic gas and supersonic ice are also great options for highly valued materials. As well as hunting missions such as EDENCOM's field bases. These options involve a great deal of risk and therefore require you to be an experienced player, ok?
For those who like action (which is also my case when I have time to play), piracy and ransom missions can be a good addition to your assets. For this style, it's important to define the group well so that the method is as profitable as possible.
I've talked about methods that involve effort and strategy, haven't I? And investment? This is the easiest way to accumulate ISK. Buy PLEX (real game money). It sounds like a joke, but it really is the best way to make a profit, and what's more, it can help you with the other forms of profitability I've mentioned!