Part one is always check the truck is out of gear before you start it. Sounds simple but it's very tempting just to reach in and hit the key. Rather than have to try and climb in to reach a gear stick. Especially if you've got dirty overalls on and you have to take those off before you can get in.
I had a near-miss as an apprentice. I did just that. I reached in hit the key without checking if it was out of gear. The truck launched forward and almost pinned one of the other boys to the truck that was parked in front of me. Almost squashing one of my mates was a hard way to learn that lesson but I've never started a truck in gear again. After tilting the cab if it's a cab over truck you need to be specifically careful because when you tilt the cab the gear linkage cables can pull it into gear. So even though you had it in neutral when you drove it in, it might not be in neutral anymore.
Part two is don't leave the keys in the ignition if you're working on the vehicle. Why? because there is some special individuals in the world they might decide to start the truck while your fingers are here checking the belt tension.
A classic one is the driver wanting to charge his phone and needs to turn the key to ignition but then accidentally turns it to start. Another scenario is they don't realize you're underneath working on the vehicle and they decide to start the truck up and drive away. Don't let this be the last thing you ever see. So chuck the keys in your pocket and at the very least don't let anyone sit in e driver's seat while you're working on it