I am a big fan of Andy Collins and his work on the Magic Item Compendium (3.5 D&D) for Wizards of the Coast. I think if your designing magical items you should read his article HERE!
The big things I took away from it and always try to keep in mind are.
1. Magic Item prices need to be based on comparative analysis with the Big Six items that players always take, not formulas. For example would you buy this item or would you rather invest in a better +1 being added to your weapon/armor/shield/ring of protection/cloak of resistance/bracers of armor/ amulet of natural armor?
2. I try when possible to make items useful at all levels of play or the item can be improved by investing in it. (Room to Grow)
3. I try to stay away from the popular item slots when making something so someone will actually use the item.
4. I try to keep the fiddly bits which make items too complex to be useful to a minimum.
5. activation costs all magic items should either be activated out of combat or if they are combat potential items they should be no activation costs, free actions, swift or immediate actions. Players are not going to waste a standard action or more on most items.
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