I never thought about this before but you can make more money by refining some of your loot instead of selling it! This is done by calculating the sell price of each mineral that is used in the items. If this number is greater than the best buy offer for the item, reprocess that item and sell the minerals instead!
There's a great program (only for Windows) called EveRefinery that can help you make the choice between reprocessing and selling your items. The only thing I don't like is the choice of colouring; there's usually a lot of red and green rows and it's just too bright heh. Otherwise it works well and I've made a little more ISK than I would have before using the program. Every ISK helps!
There's a website for loot refining made by Eve Online Market Guide. You only have to supply a limited-access (asset listing permission required) API key. I haven't used this tool so I don't know how reputable it is, but with a limited-access API key they can't do any damage to your account, so why not try it.