1. Be among the first few customers to get the product you want on the boxing day as there may be limited stocks
Check and compare the offers if you are buying from online stores
2. Know your credit card limits before using the card to buy any product
3. Make sure your debit card has enough cash for your shopping, if not deposit amount yo your card one day before
Purchase the products only if you are in need, don't waste money on products which you don't use, even it has got huge discounts

(Source: http://australiaonlinenews.com/best-boxing-day-sales-deals-online-2014-australia-183/)