I also agree that offering a no quibble money back guarantee benefits both customer and seller, I also believe in seriously over delivering value and service. But there is one tactic that you could use which is this.

Let's say you offering a 60 day money back guarantee, you could have your autoresponder setup that on the day 55 your customer will receive an unexpected and extremely valuable bonus within seven days, you could even start a bit before that building the unexpected bonus up and a few days before the guarantee is up, announce that they will receive it free of charge for being a valued customer.

Using this tactic should lessen your refunds, but you will always get a few who want a refund regardless of what you offer them.

Just a thought,
Lee