Well, I did some more research and consulted my feng-shui books, and can tell you that whatever plant you choose should have rounded leaves, definitely not anything spiky like a cactus. The round shape attracts positive energy, whereas the spiky thorns of the cactus cause energy to flow right out, draining a room. Also, plants that flower are good choices, as they bring more energy in as they blossom. Upkeep, however, is essential. Dead flowers and stems should be cut immediately, as they drain energy.
I think lavender and mint are both good choices. Another to consider might be a jade plant, which is supposed to bring luck and wealth, especially when placed in the back left corner of the room (when looking in the door).
I've been studying feng-shui lately, and though I am by no means an expert, if you had any other questions I would try to help.