Depending on which version of PT you are using you may have up to 10 "Price Levels" avail to you when setting up inventory items. Enter prices in these ten different price levels that correspond to a discounted price. For example, PriceList1 might be your list price. PriceList2 is 5% discount. PriceList3 is 10% discount, etc. etc.
Then when you setup your customer, go to the window titled "Sales Defaults". On this window is a field where you choose the default Price Level for that customer. If you choose Price Level 2 then all the prices shown on an invoice created for that customer will reflect a 5% discount (as per my example).
Hope this helps, reply if you have any questions.