I make it very clear that I have no intention of flooding them with spam and give them the choice of the number of times they will be contacted in a month. This has two effects, they gain trust in your professionalism as you are seen not to be just harvesting emails and it also makes them feel in control as they set the frequency of contact.
It does mean that I have to operate a number of mailing lists but this is a small price to pay to lock customers into my products.
The majority of my customers (approx. 90%) join the '1-4 times a month list which is great as this means that I can contact them once a week with offers if I needed. This is a frequency that I feel is adequate to maintain the relationship between me and my customers as I am not seen to be spamming them but still seen to be offering products of interest to them.
It is also important to note that I do not always send out one mailing a week as it is dependent on the product you sell as to whether the customer would want to purchase once a week. i believe greatly in the relationship that I have with my customers and can see the long term benefits of maintaining this and therefore I refuse to jepordise it for a quick profit.
A point to remember, if they choose not to take the opportunity to join your list then do not keep emailing them. this is SPAM and will do no more than tarnish your company, website and products image in the eye of the buyer.
One of the next things that I found was that as my lists grew I was spending more and more time emailing and I looked to a solution to help me automate this part of my business so that I could spend more time attracting customers, after all they provide me with my profit.
Again the simple answer was an Autoresponder, this tool allows you to automate the email process for your lists and can be setup to manage such things as information requests so that the customer receives an almost instant response to general queries without you having to type a reply.
I have tried a number of tools through the years but have found that the list software that I recommended above offer a great combined service and high mail delivery %'s. I have also recently had the following recommended to me, as of yet I haven't tried it but I will be in the near future as they are offerring a unique service to incrase your mailing list numbers.