Your feedback is your biggest selling tool bar none and you must protect this the best way you can.
I have found through experience being polite at all times and in all circumstances goes a very long way in achieving a good rating.
A happy customer is a good customer and you will find many eBayers come back for more if you are polite and efficient.

Always be polite in any emails with customers particularly before any sale.

Do your best to reply to any questions within 24 hours. If you have the time check your emails as often as you can as a potential customer will only look somewhere else if you don't respond quickly. If you do reply after 24 hours start your email by apologising for the delay. If you can set up an auto responder letting your potential customer know your email has been received.

When I send out my email to the winning bidder I always have it look something like:-

Dear eBay Member.
Thank you so much for choosing to buy from me.
I know you have a lot of choices and I appreciate your business.
First of all, if you are not completely satisfied with your purchase from me please let me know before leaving feedback.
As long as you are happy with your purchase please leave me positive feedback and I will do the same for you as soon as I can.
If you have not checked out my eBay store why not take a look as I have many more quality items for sale.
(Enter the link to your eBay store here)
Once again thank you, I hope we can do business again in the future.
Regards.
John

Straight away the transaction is off to a friendly start.
I feel this politeness only encourages the customer to leave nothing but positive feedback as I have stated I will do the same for them. Also, they have been made aware to contact me first before leaving any feedback if there are any problems or concerns. Notice how I have thanked them twice for their custom and hope to do business again. This encourages them to consider further purchases. Perhaps even clicking on the link to my eBay store and buying more items.

From time to time you will get awkward offensive customers but try to rise above any war of words and stay calm. If you have to reply always be polite. I know it can be hard but if you stay polite while someone else is losing it you will feel better.

There will always be one awkward eBayer who leaves a negative, this will happen no matter how good you are and my experience has found it's normally the newbies who are most likely to leave you a negative, this is also normally done without contacting you first and it can be very frustrating. Be prepared as your first neg is the hardest to take.

One or two negatives per 500 or so sales won't hurt anyway and if you keep your percentage above 98-99% you will have no problems.

Just remember. ALWAYS BE POLITE. Manners cost nothing and it helps make eBay a better place to trade.