@Nick's comments cover all of the bases, but do you really need to offer discounts?
Maybe you could make clear that you provide free consultations or are willing to spend a certain amount of time on a pitch/proposal on a 'no-win/no-fee' basis. You could also offer your customers 'back-handers' for getting new business from another company for you, however, 'back-handers' will need to be sold a bit fluffier than that even if it amounts to the same thing.
For introduction, you can write articles that focus on trends in graphics/web design plus topics that always interest people, e.g. what new colours are 'in' for the next season. Hopefully such content will encourage your customers to be inspired, maybe inspired enough to come to you with some new work.
There is no harm in repeating what-your-basic-business-offering is. Hopefully you understand the 'pain' of your customers and offer them 'pain-free' ways to achieve their goals. Learn to understand what most people hate about how website projects are delivered and write with a pre-emptive response to that. Truth is that most companies have a loath-hate relationship with 'web developers' so there are plenty of opportunities out there for the picking.
Must-have technological extras that play on 'FUD' could also be a sales 'in'. Adding microformats to a site is a very good idea if you want sensible search rankings, SPF records on emails to make sure emails get read is another idea, albeit outside the 'design remit'.
This sort of content widens your newsletter to a business audience, these smaller tasks also give your customers an opportunity to work with you on something that is manageable and not of great risk. If you can turn around a client's email 'always ending up in SPAM' they will be grateful to you and you will have that element of trust. You will also show you don't just do pretty web-pages, you also can do things more pertinent to their business needs.
Through delivering one of your normal design projects, you will learn and master a topic or two. At the end you can write it up succinctly as only someone that understands their stuff can. If you can then confer this gift of knowledge onto your customer base then the penny will drop, they may understand a topic that has eluded them for some time.
An example topic is site migration, what needs doing before and after go-live. Again, this will make the impossible (moving website) seem possible.
With your language you can talk up to your audience, to 'raise their intellect' during the reading of your newsletter, not to talk down, or dumb down. Note that the best writing does not use the biggest words, so hone your prose and make sure there are no errors.
I imagine you will be wanting to impress with design. Get your newsletter looking the part so that your abilities in this area are implicitly stated.
It goes without saying that nobody will read your newsletter if the subject is no good. The subject is only there to get people to open your newsletter, it does not have to have any relevance to the content, so long as you qualify it somehow. You need not stoop to tabloid style headlines (e.g. "Freddie Starr Ate My Hamster") but you should be thinking on the same lines - what is going to make people want to read this?