I use vaccum bags (the ones that don't actually need a vaccum though.) You can either roll them or sit/jump on them to squeeze the air out and I've found that they really work well for me. I've heard horror stories of people having to unpack entire suitcases at the airport, or everything getting soaked when it was left out in the rain, so everything that goes into my bag is either in a vaccum bag or a zip top bag. (that also protects everything from leaking bottles of shampoo!)
If I'm going on vacation I try to bring full-sized, nearly-empty bottles of shampoo/conditioner. This way they take up more space when I'm going on vacation (so nothing is flying around inside my suitcase) but by the time I'm coming back I've used it all up and binned the bottle, giving me more space for souvenirs.
I don't use a lock at airports. They'll cut it off if they want to look inside your bag, and anything that hangs off of your bag like that can get caught on something and tear your bag. One suggestion I've heard was to use those plastic cable zip ties (does anyone know what I'm talking about? You put one end through the hole on the other end and you can tighten it but you can't loosen it?) If nothing else it will slow someone down a bit. I DO use locks though (I have 4! And only heavy-duty ones) anytime I leave my bag where any other people might be able to get at it eg; luggage room, train, dodgy hotel room, -anywhere you wouldn't leave your passport.