Just yesterday SixRevisions wrote 10 great tips to write email newsletter, even though I disagree on some points it is still a great read. In this post I'll expand a bit on some of the points in the original article in terms of marketing or sales pitch via email.
#2 first impression
I don't think using HTML email for marketing, unless you really have to put in some of the product images, the HTML should still look like plain text email. A full-on HTML email with 2-3 column layout just alerting viewers first spam alert.
#3 using your brand
Putting identifiable from name, and a logo in the signature should be more than enough. A full navigation from your site inside email is just adding more noise and distraction from your intended pitch.
#5 personalise email messages
I think this should be taken to a step further. Create a personalised url, http://website.com/this-email-title/john-smith, this way you can quickly see who's interested with your pitch, in what demographic is that user? Are more people in that demographic responded as well with this pitch? And should I start placing ads in magazines with the particular demographic as target market.
#8 content is king
As described in http://four.sentenc.es/ email contents should be treated like SMS. Short and clear. 1 paragraph containing your elevator pitch with a link (read above) that explains more about the pitch/product. If you are just sending out news or blog post instead of marketing pitch, it should be just the excerpt of the post.
#11 put some time between mail-outs and don't pitch the same crap over and over and over again. If you are to send email every week, you should let your customer know and have them re-confirm when you're importing their email (there's an option to choose when you're importing email to campaign monitor). I've seen a company shrunk their database from 10,000 to 8,000 in 2 months, including a few nasty replies and 1 angry phone call from a customer.