From someone who travels a lot with knives and other gear not allowed in carry on. Secure your luggage with a vividly colored/distinctive cable tie (not plain white tie that's easy to substitute). Take a dated digital image, preferably with the baggage tag included. (camera in phone works adequate for this, but I often use the Digital Elph I carry).
If the TSA opens the bag for inspection, they will usually secure it again with one of their seals. In order to steal anything from it, the baggage handlers have to break the seal. They have easier bags to steal from, so why bother? If they do and something is missing, you have some degree of proof it was the airline (more or less). If the bag comes off the plane without the cable tie or a TSA seal, you know something is likely amiss and
immediately inspect the bag and file for a loss right then and there. It will carry a good deal more weight with the airline. It is, of course, a PITA and time consuming.
I carry a large pair of toe-nail clippers to cut the cable ties (and spare cable ties for trip home or next flight in my computer bag), doesn't raise any issues with TSA at this point.
If it's more valuable stuff, do it like we do firearms. Use a hard case with a padlock. Get there early, wait while they inspect the bag, then lock it afterwards. If really paranoid, get a cheap firearm to place in it and declare it as well. Just be really careful where you travel with it.
If they are really valuable, send them FedEx or UPS fully insured. With UPS, they now require the driver to sign for high declared value stuff and keep a pretty close tabs on it.