First, you don't want to get the aforementioned crud on your clothes. In an opposite world to department stores, you will also likely get treated better by the staff, get the scoop on upcoming sales, etc. Leave your friend at home. He or she will just slow you down. You'll also avoid arguments if you don't both simultaneously dive for that great '50s double-helix lamp shade.
Some stores have special sections for extra large clothing, maternity and the like.
Furniture is often in the basement or the back of the store. The signs near the register tell you what is on sale that day but sometimes deciphering them isn't worth the effort unless its a big ticket item such as furniture. When you're ready to pay, estimate the price of all your items based on the tags and the signs, and then smile happily when the cashier gives you change. Bring your patience and good manners. The actual checkout process can also be slow because of the previously mentioned sign confusion, language differences between customers and employees, older cash registers and even older patrons.