a couple of suggestions... go to the university (complutense is close and a good place to start) usually you'll see postings from other people looking for flats. La facultad de Fiologia is a good place to start (its right next to derecho and its where they give spanish classes to foreigners).
Also, look in Segundamano, it comes out 3 times a week, it also posts announcements for shared flats. You can also check it out online: www.segundamano.es
STAY AWAY from places that when you call about an offer and they end up you going to an office and for a fee will find a place for you at what seems to be too good to be true rents! Its a brokerage firm and they're only profiting off of you AND getting you to spend more money than you need too.
Also, I know this sounds so cliche, but at most irish pubs, Chesterfields for example, they have a free newspaper for English speakers and you may get some leads from there.
There is a place called the Mercado Fuencarral on the C/fuencarral, that lots of young people go to (its full of shops- artisan work and more!) and there are bulliten boards with posts and you may also get something from there.
If I were you, I'd start with segundamano. But be warned! Places go fast, they find someone they like and bam! Its theirs! So check it out early and start calling to check out the apartments/potential roomates!