For my MOCs, I like to build modular city buildings. This is my strategy: First, buy two of every modular building (one for display, and one for my brick collection). Second, buy one of each Creator house to add to my brick collection (I find that the Creator homes are a great source of bricks, but I don't find them worthy of keeping on display). Third, start branching out into other themes that you think have useful parts (I think the winter village sets are great). Fourth, if you are lucky enough to have a LEGO store nearby, then you can visit periodically to load up on pick-a-brick parts. Last, to pick up those odd pieces you need to finish a particular project, use Brick-Link.
LEGO is an expensive hobby when you are starting from scratch, because you need to buy lots of sets to build up your personal brick inventory. However, as your collection gets bigger, you won't need to buy as many sets out of necessity (you'll just buy them because they are awesome!).