The simplest way to avoid checked bag fees is to not check a bag, and that's more doable than it sounds. Most of the big US carriers raised their checked-bag prices again this year, so the fee that used to feel like a rounding error now shows up as a real line on the receipt. A second bag usually costs more than the first, and prepaying online is cheaper than paying at the airport counter, but the biggest savings come from skipping the fee entirely.
Pack to the carry-on limit
On a standard economy fare you almost always get a carry-on bag in the overhead plus a personal item under the seat. That's two pieces of luggage before anyone charges you a cent. A lot of trips that people instinctively check for actually fit in a carry-on if you pack once and don't over-buy.
Two things to watch. First, size and weight limits are real, and gate agents do check when a flight is full. Measure your bag against the airline's stated dimensions instead of trusting that it "usually fits." Second, if the plane is packed and the overheads fill up, a carry-on can get gate-checked anyway, which is free but means waiting at the carousel like everyone else.
When a bag included is the cheaper ticket
The trap is buying the lowest fare and adding the bag after. On many airlines the cheapest fare is basic economy, and that fare often doesn't include a carry-on at all, only the personal item. Add a bag to it and you can end up paying more than a standard fare that bundled the luggage in from the start.
So compare the real total, not the sticker. This is the same math behind the hidden cost of a cheap fare: price the trip with the bags you'll actually bring, then pick the fare. Sometimes the "expensive" ticket is the cheaper one once the bag is in.
When a card or status actually pays for it
An airline credit card that includes a free first checked bag can be worth it, but only if you fly that airline enough for the free bags to clear the annual fee. Elite status does the same thing without the fee, if you have it. Neither is a reason to sign up for something you wouldn't otherwise use.
When you're weighing the three fares we send each morning through Flight Watcher, factor the bag in before you decide. The lowest number on the screen isn't always the lowest number you'll pay.