Here's how I use email for promotion:
1. Send promotional emails to clients who you know and who will appreciate receiving them. After all, what's the point of engendering ill will by abusing people you've never even met?
2. Do it with the kind of limited frequency that will prevent you from wearing out your welcome. Once every three months works for me and gives me enough time to collect enough new work that I'm really happy to show.
3. Keep it simple, elegant and to the point.
Here are some examples of campaigns going back over the past few years: