I normally run a search on the website below. If you register with this site, they run a loyalty scheme (which gives a 10% discount once enough points have been built up).
The best thing about registering is that you can see which car hire companies the bookings will be with upfront. So you can pick and choose the best company (e.g. I found it was always best to avoid Gold Car at Murcia San Javier Airport as the queue was always huge, or you can avoid companies which have their base outside the airports e.g. Dickmanns at Alicante).
http://www.economycarhire.com/?gclid=CJvz_4mKoK4CFQcRfAod_m9m7A