No, there's not an awful lot around about decorating rentals, is there? And a lot of it is "well, maybe they WILL let you paint..." which isn't helpful because usually they won't, and why go to all that effort to improve someone else's investment?
I'm renting a beige box at the moment, and not only is the house itself pretty blah, but the furniture's weird too. I took over the lease from a friend who was moving - she couldn't take all her stuff to the new place and I don't own much furniture, so I'm using a lot of her bigger items on an open-ended loan. So for instance, I've got her very sleek and modern dining table in light beech paired with my fancy old silky oak office chairs!
The first thing I did was take down the nasty lace curtains that came with the place (and stored them away so I can put them up again when I go) and put up some bright, fresh ones of my own. I used floor-length ones, so it's quite a big slab of colour and really makes a difference to the room. Friend also gifted me a huge bright red rug, which does the trick too.
I'm very lucky in that most of my rooms have picture rails, so I've gone nuts hanging things on the walls. Not just pictures but a flag, dotee dolls from a swap and some of my small musical instruments. For my bedroom I made some quick and easy bunting using scrapbooking paper from my stash, and strung that all around the walls too. (Yes, I am mentally about four!)
At least with beige you can drown it out with bright accessories. For instance in the kitchen I've got two vintage trays on the wall with blue in their design, mostly blue-toned teatowels, gold-lidded storage jars in open shelves, and some gold-white-and-blue fish ornaments on top of the fridge. The space is tiny so that's all it needs to have an overall blue-and gold effect, rather than beige.
I hope that's some sort of starting point... sorry to ramble!
Edited to add...
I also have a few large framed posters that I'm not game to hang, because they're quite heavy and I don't know how much the picture rails can hold. So I've got them leaning against walls in random places - to fill in large blank areas of wall, or hide some of the crappy repairs that have been done over the years. There's also nasty old non-functioning radiator in one corner of the lounge room, which I've hidden by putting the couch in front of it. It cuts down the amount of space in the room, because the couch is 3ft further forwards than it would otherwise be, but I don't have to look at that piece of junk.