The best thing to do to be successful on Ebay is to be honest.  You may have heard of sellers who over inflate shipping charges to cover low bids and pay less in fees, not only is the against Ebay policy, your buyers aren't going to like it.  You WILL lose customers and gain plenty of NEGATIVE feedback.   Here are some tips to keep your rating at 100% and have return buyers.  

• Keep your listings as accurate as possible do not try to be misleading in your listings, false ads lead to charge backs, disputes, and negative feedback.   Make sure you know the product you or selling.
• Use accurate postage fees, for easiest shipping the postal service has FREE flat rate priority shipping envelopes and boxes. You can buy and print your postage/ mailing lables from Paypal and get free delivery confirmation! You can also schedule free postal pickups.  If you can't or don't want to use the flat rate, I highly reccomend purchasing a small scale.    If you must add a handeling charge only use it to cover your actual shipping material costs.   Do not charge $10 to mail a shirt.  Offer combined shipping to encourage multiple purchases.
• Have a clear shipping, payment, and return policy listed in every ad.  Make sure you explain what payments you accept.  If you don't already have one I highly recommend a Paypal account.  Many buyers will pay instantly through Paypal.
• Ship as soon as possible after the payment clears.  When possible a seller should ship within 24-48 hours.   If you can only ship one day a week, clearly state that in your auction.
• Encourage customers to contact you with concerns or problems.   If a buyer finds a problem with the item that you did not notice, do everything in your power to make it right, for something minor offer a store credit or partial refund.   For a major problem have the buyer return the item and refund their money.   For something major that you may have missed I would also refund their shipping charges too.  I would rather lose a few dollars than risk getting a neg feedback or losing a customer. 
• When all else fails give the buyer the benifit of the doubt, check their feedback if it is full of problems with sellers and you honestly believe there was no problem with the item then you may have to just accept the fact and you may get a neg.
•  IF you get a NEG:  DO NOT GET DEFENSIVE  nothing will turn a buyer off faster then reading name calling, and bad mouthing from a seller in the feedback.  Simply state the facts, if necessary add "please view buyer's feedback" if they have a history of being difficult it will show. 
• TREAT A BUYER AS YOU WOULD WANT TO BE TREATED!