1.) Buy a Premium Ticket
Booking an economy ticket guarantees you a seat on the plane but not much more. One of the ways to get around being charged for your first or second checked bag is to upgrade to a premium ticket.
Most major airlines offer several tiers of airline tickets that start from economy basic and go to premium first class. When you purchase a ticket in a class above the basic economy, your premium ticket will usually come with a free checked bag.
The highest levels of tickets will offer multiple checked bags.