Power has a price.
With Great Power, comes Great Responsibility.
There's no such thing as a free lunch.
We're all familiar with these platitudes. More, we try to apply them to the magics and items we introduce into the game, and the prices on magic come in three major categories: Curses, Consequences, and Conditions.
The most abused and misunderstood of all the classes of prices, the curse is still a time honored tradition in roleplay. A proper curse is a deliberate and malignant addition to a magic, for one of several possible purposes: Protection, vengance, sheer hatred, insurance, etc. Curses should be universally detrimental to the user of the magic. Curses should also have a 'way out', though this should also involve losing any benefits of the item.
Quick examples of curses: A mystical cloak which inflicts wanderlust upon its victim. A blade which cuts its wielder.
Most 'Curses' are mislabed consequences. A consequence is a logical outgrowth of the magic's action, and should be the most common form of price on the magic. To create a consequence, consider the magic that forms the beneficial effect of the spell, and how it reaches that benefit.
As our example, we will consider a simple ring which grants 'haste' to the wearer. We know that our ring will cause our character to move faster in battle. We decide that it does so by enhancing his metabolism, to produce more energy, and the speed at which his nervous system reacts. As the wearer's metabolism increases, her body temperature rises. Calories from food are consumed faster. Wearing the ring for too long, then, places the user into a fevered state, possibly even approaching an illness. The wearer of the ring must eat more often. As his nervous system is 'too fast', he cannot check impulse and instictive actions. He is twitchy, and abnormally likely to attack someone who suprises him, even in social situations.
Conditions are exactly that: Something which needs to be fufilled before an item can be used. Commonly, these are attached to divinely based items: One must be a worshipper of the God before He permits the use of his power. This may also be applied to items which 'replace' body parts: You've got to cut your own off, first, which may involve varying degrees of sacrifice and pain. To create a condition is much like creating a consequence: Consider who created the item, and what it was meant to do. Then, bring the body / mind / soul of the user into alignment with the creator or purpose of the item, in some fashion.
The classical example of a condition on an item is the Hand of Vecna: One must remove one's own hand to attach the Hand of Vecna. Of course, this also has plenty of consequences attached to the Hand, but these are seperate from the condition of use.
Power has a price. Make sure you choose your coin well.