If you're embarking on a venture that will involve frequent shipments, it's a good idea to set up an account with the company or companies you use most often. Some companies will give you a discount simply for setting up an account; others will give you access to free supplies.
Try to go to the same office or counter each time, and build a friendly relationship with the clerks. Don't waste their time; have your shipment ready to go when you get there.
Save yourself time by printing professional labels (or, more frugally, buying a self-inking stamp) with your return address information. Also, include a business card or receipt with your name and address inside each package in case the label is damaged.
Don't skimp on tape. A continuous wrap around the package provides a substantial reinforcement for minimal cost.