1. Make sure you have a great list. By "great" I mean targeted to the specific demographic and psychographic profile of your market.
2. Make sure you have a great message and offer.
3. If you aren't a skilled copywriter - hire one. Consider your cost of client acquisition (and if you haven't calculated your cost of acquisition - you need to before you do ANY marketing) when deciding on how much you can afford.
4. Split test the mailing so you can find your control.
5. Measure everything.
Give me a call if I can help with more on the specifics.
And good luck!