Some airlines let you purchase additional baggage in advance, which is typically cheaper than paying for the same bag at the airport and will save you time.
Check in online for your flight as soon as you can, as your chances of getting bumped off will decrease, and sometimes, you'll be able to choose your seat for free.
Driving to the airport is the most convenient option, but traffic can hold you up, and airport parking can be pricey. Take public transport to the airport where possible.
Airlines have strict cut-off times for each step of the process, so try to arrive two hours before a domestic flight and three hours before an international flight.