What accreditations would you likely place on your web site to get that quality assurance across to visitors?
I'm thinking of things like GAWDS or Google Adwords Member.
Are there any others I should consider?
Quality assurance doesn't come from joining guilds and organisations, there's a whole bunch of them out there and very few of them actually check to make sure those who are members do meet the guidelines set out (and actually - none that I'm aware of pro-actively check members post joining so it's not a guarantee mark). The problem is that your clients are unlikely to know who these organisations are or what your membership means, therefore they are not going to be impressed by you being a member of some group which could be a good thing (or not). The only justification for joining a guild or organisation like the many which exist is if you want to pro-actively place standards upon the work you undertake and / or actively promote the goals of the group in question. As an example - it's great to promote web standards and it's fine to say that you follow W3C validators - but the average client and Internet user doesn't even know what the W3C is and they won't be impressed by a "pass" message on your code. You are effectively asking the average person to be an expert like you, which just doesn't work in the real world.
I should disclose at this point that I am a member of both GAWDS (Guild of Accessible Web Designers) and the IWA-HWG (International Webmasters Association & HTML Writers Guild). I joined in order to pro-actively promote best practices and accessibility standards within work and writing (which I am very passionate about).
Examples of your previous work and genuine testimonials, if you have any. And your website should be very functional, very usable, original and attractive. It seems to me that those are the only real qualifications you need. I don't think anyone will care you're a Google Adwords Member, let alone know what it is (I don't, for example).
Totally agree on the Testimonials front. They can't be beat.
I'm sure I've seen sites in the past really "pushing" the fact they're passing W3C standards. I normally leave these out as it tends to confuse users a bit. And I'll try make the sites validate anyway
Are there any other standards/guilds you can think of?