How to avoid carry-on luggage fees on budget airlines

Certain airlines offer very low prices on flights, and then charge for things most airlines include in the ticket price, like carry-on luggage. On Frontier or Spirit, a carry-on bag is $35-$60, each way. I avoid the fee by using luggage that is small enough to be considered a "personal item" (It has to be under 14" high, 18" wide, 8" deep). A lot of backpacks will meet the size requirements. I have a Swissgear rolling bag, similar to this one. It holds enough summer clothes for a 4-night trip, plus a gadget case, a toiletries bag, and my 13-inch laptop with charger.