I'd say a key to that is to make sure you focus on giving pertinent information in your "SPAM", and only put a line on what you're trying to sell and let people go further if they wish. That way, next time, they'll open your email again...
I say this because I used to receive stuff from Salad.com (something about comminucation). They used to send a Tip Of The Week (like "Improving your sales", "Better Web Marketing") with some quite valuable information. Then at the end they put some links to commercial material (videos, books, trainings). I nearly bought something once. Then somehow, this changed, and they used to put less and less info and more and more SPAM, so I finally SPAMMED them. I thought it was a shame, because I liked their Tip Of The Week, and I really did intend on buying something at some point.
Chris