Wait to upgrade
If you wait until the day of your flight, you can often pay less for an upgrade, as they are incentivized to fill those seats. Prices often fluctuate depending on whether the flight is considered short, medium, or long, but I often find that medium-length flights (around three hours) are the best deal to upgrade. Don't bother with two hours or less: you can suck it up for those.
Remember these three things about luggage
1. If you are going on a trip that is 48 hours or less, you should be able to fit everything you need in a smartly designed backpack or small duffel that could, if need be, fit under your seat (plus a hanging bag that most airlines will take and stow for you if you ask).