Answer:
Legally, you can't.
EDIT: You can't if you are supposed to. You can either not buy the item or purchase it from a state that doesn't impose a sales tax, but keep in mind many states also have a "use tax" which is a reciprocal of sales tax - (don't laugh...) you're supposed to voluntarily submit the sales tax that SHOULD have been collected if the vendor did not charge you for it. Pennsylvania is one state that has Sales and Use tax laws.
Certain states permit exemptions from paying sales taxes... for example many states exclude equipment, materials, etc used in farming operations from being taxable. In those cases you would fill out a form from the state and give it to the vendor to inform them that you are exempt and also to give them the necessary information to defend their position of not collecting the tax in the event they get audited. States enforce sales taxes by periodically auditing the licensed collectors. If it is determined that the collector has not been collecting the tax as required, the collector can be forced to pay the taxes that should have been collected, or if the amounts are significant or if you provided false exeption information, the state can come back to the pruchaser.
Note: Purchase your expensive items online to avoid sales tax...Maryland has a 7% sales tax..if you buy a $400 Ipod you will pay 28 dollars in sales tax,but if you order it online you pay no Maryland sales tax and only $5.00 for shipping.