I've raved about how awesome flowers and indoor gardens and outdoor havens really are before (hanging gardens here, succulents here and here), but really. Get yo'self some plants, or at least dump some flowers in a vase-like-object, and see how much better your place looks!
Some tips and helpful info:
1) Kill every plant that comes under your roof? Get succulents, cacti, or (my favorite) orchids. You seriously can dip an orchids into water once a month and they'll love you for it.
My own succulents, in DIY'ed tiny pots
2) Here's a really cool website about houseplants that help clean your air
3) And of course, Apartment Therapy (quickly becoming one of my favorite websites ever) has this great list of how you can DIY your own indoor window box
4) Plants don't have to be overwhelming. Two white roses in a tiny clear vase on a bookshelf is beautiful too!
5) Have fun and be creative with it! Flowers don't have to be pink and girly - add some action figures for extra toughness, or stick with simple greens in indoor plant boxes!