There is a lot of risk involved when buying a service. And web design is a service. Countless sites fail due to poor design, unmet needs, unidentified markets, no publicity, poor search engine rankings, long download times... You name it, hundreds have failed because of it. You need to be aware of these risks and minimize them as much as possible. So what can you do?
Identify your needs! This simple approach is so often ignored. It is vital to educate yourself of the possibilities so that you can define your needs. Don't go to a design firm unprepared. You need to understand what you want your site to do! This means writing a detailed brief. Don't consider any designers until you have done this.
When you start looking at designers, here are some things to consider...
Check their previous successes. Speak to their clients. Ask for testimonials.
Do they ask about your needs! Information is important. A good design firm will be interested in your needs and will ask endless questions about your company, your marketing requirements, your customers, your product or service. If they only talk about appearances, look elsewhere.
Are they easy to work with?
Can you contact them? Do they reply promptly? Do they give honest answers that inform and benefit you?
This is where self education is important. Finding out the technical implications for your site isn't difficult. Post a question on a web development forum, read a tutorial, ask around. You'll soon find out what the basics are. Then its a matter of asking the designer if he can provide those skills. For the future of web development, meeting web standards and cross browser support is a given. If they don't understand this, look elsewhere.
If you aren't satisfied, what have you got to fall back on. If they don't stand behind their service, look elsewhere.
For a fantastic collection of indepth development parctises and web related articles visit Aleksandar Vacic's site "aplus".