When boarding a full plane, how can I guarantee that I can keep my handheld baggage with me?

If your handheld bag is small enough to go under the seat in front of you, it won't be taken from you. If you want to bring a large roll-a-bord to save checking a bag, you can do that, but keep the things you genuinely need during the flight in a separate smaller bag. Also, do not choose a seat, such as the first seat in economy, that doesn't have a "seat in ..