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You cannot beat the personal touch - pick up the phone, or at least send an email addressed personally to the member. Follow up is important too. Remember to say thanks. Always respond to an email - don't leave them wondering if you ever received it - even if you all you say is "I will get back to you tomorrow".
Can you offer an incentive or several different incentives targeted at different groups? Know what your 'wriggle room' limits are.
Be kind, be gentle and remember the customer (aka your member) is highly valuable to you and is "always right" even if they are not!
This is what I do
What do you do to promote your product to your members. What works the best?