I usually do it all myself - just because my budget is quite limited.
However there are some books that might be worth reading:
- Homepage Usability - 50 Websites Deconstructed by Jakob Nielsen & Marie Tahir
- Don't Make Me Think by Steve Krug
- Designing Interfaces by Jenifer Tidwell
- Prioritizing Web Usability by Jakob Nielsen & Hoa Loranger
And it's always a good thing to look what other sites do that you like :)
