Rule #2: Make them feel special. Send them a decent thank you email or even better, send them a seperate pdf in their downloads which is a thank you letter.
Rule #3: Help them digest the product. Make it as easy for them as possible to implement. If your product works, you'll have raving fans for life. Many a time I have just given up on products which were just to difficult to work out.
When writing products, do it in a way that a 5 year old can understand. Never assume that every customer has a high IQ! Where practical, always include a fast-start guide that explains each step of the journey.
Rule #4: Make them feel like family. This applies only / usually if you sell a product based on your expertise. For example, my first digital products were about real estate based on my own success, so in this case I provided a forum for customers. I made them feel a part of an exclusive community / club.
Do not underestimate this aspect. Sure, it takes time and effort, but it not only cuts refunds, but also increases the lifetime value of your customers.
Rule #5: Be Accessible! I don't care if you sell a $7 report or a $1997 home study course. What is only $7 to you may be a week's wages for your customer. So make an effort to support their questions.
If you don't want to do that, then at least make up a FAQ page in advance. This not only cuts down your customer service, but it also better qualifies your buyer in the first place. The FAQ will help overcome most objections before they buy.
BONUS Rule #6: Don't sell cr@p. Self explanatory.
Feel free to add your own suggestions.
Best of success,