I know this has been mentioned before but it bears repeating.
Keep in touch with your clients regularly using a dedicated bulk email program. I personally use Constant Contact for both the salon and the distribution company, but there are free ones available like Mail Chimp.
I send out no more than 2 a month and without exception it brings in at least two new appointments.
What do I put into them? I might choose something topical like the weather e.g. "your skin will suffer in this cold weather; you might want to consider xxxxx cream which is nourishing and protective"
Even if people don't take up the special offer, or whichever treatment/product I'm promoting what it does do is prompt people to make an appointment anyway, "aha I've been meaning to ring and this just jogged my memory!"
Whichever program you use don't make the mistake of using your normal email software like outlook, hotmail etc when sending out bulk email. It will likely get tagged as spam, and could land you in bother with your ISP.
If you haven't been in touch with your clients lately jump to it! If you're at a loss as to what to say in an email to them drop me a pm with your email address and I'll add you to our lists and you can see what we say to our clients!
Totally agree on everything Lynne has said and using your 'local' email isn't a good thing to do as it can land you in all sorts of hot water. A recognised email comm's provider such as MailChimp is good and I use AWeber, which is another good system (and easy to use) and the pricing isn't bad at $USD20 per month. The key, I've found, is regular communications. Whether it's weekly or twice per month, try and do it on the same day at the same time of any given week. Clients actually do come to expect it and miss it when it doesn't arrive. I've had situations where clients will phone me and ask if they've missed it because they are keen to see the specials etc.