Create a spreadsheet for keeping track of your shipping dates. Even if you don't ship things very frequently, the spreadsheet will help keep you organized. By referring to the spreadsheet, you'll have an idea of what needs to be sent out before it gets too late. That way, you can avoid the high price of overnight or two-day shipping.
Reuse boxes and other packing materials that you already have around the house. This will keep you from having to buy these items at the shipping counter.
Take an extra moment to compare shipping rates before you choose one carrier. Do this by using the carrier's online shipping fee estimator or by visiting each one in person. Shopping around for the cheapest rates will save you money every time.
Place as many items as possible into a single package or container. Although you'll end up with one heavier shipment instead of several smaller ones, you won't have to pay the fees that are associated with sending more than one package. So, a 15 lb. box, for example, could be cheaper to ship than three 5 lb. boxes because you'll only be paying fees for a single shipment.