Even if you not have to agree with all puritanic ideas of usability guru Jakob Nielsen in his postings at his website Useit.com it’s great reading all the time. Read his last finding: “Fast, Cheap, and Good: Yes, You Can Have It All“.
Nielsen means the maxim: “…if you want something done quickly and inexpensively, it’ll be of poor quality; if you want it quickly and done well, it’ll be expensive” is not applicable “for one important aspect of usability: methodology”. The fastest and cheapest methods are the best in usability. He suggests paper prototypes and testing of each design with 5 users and to run many rounds of user testing.