I like to describe my fiancé and my room as a "punk rock gyspy caravan museum."
We are both very much into DIY projects, installation art, whimsical settings and antique furniture. As we generally can't afford the latter, we've become expert dumpster divers (always keep an eye out for great finds during council clean up on your streets), Gumtree or Craigslist finds, or things friends are throwing out. Any of these things can be painted, destroyed, reworked and made into an amazing feature piece for your room or house.
I've always believed your room is your sanctuary, a place to relax and breathe, to feel content to be within and come home to. I've also lived in many share-houses in my time, and this is no exception.
A lot of rental properties do not allow the use of nails or other permanent modifications as per the rental agreement. But I've found that there are always ways around this.
Get creative with picture rails.
In older homes these generally feature prominently — they're the high up border surrounding the room. I use these to hang coat hangers from and from these I have art works, flags, bunting, and my fiancé has even used them to put up surround sound speakers.
Blu-tack is also highly reliable.
As long as it doesn't leave oily marks, though these can be easily removed with a bit of metho (or rubbing alcohol).
Hang things from your light fixtures.
Build up, not out.
I've got cabinets on top of cabinets, shelves on top of end tables, manniquins on bookshelves, and I like to hang flags and drape fabrics because it makes the room feel warm.
This also allows maximum hoarding of your cool stuff!
I've come to learn to walk a fine line between social housing and having my own space. How much room you have is of no consequence, as long as it's yours and you make it just so.