Strongest form of IP protection is probably putting: "Support game developers!, if you like it BUY IT!" somewhere visible.
Jokes aside, while I’m no expert on the subject, here’s what I know. It’s a bit different when you develop all the stuff on your own and when you’re hiring people.
Most often, when you’ll have to protect your shit vs. copyright claims, it’s either some wanker trying to mooch you for money - in this case you need to be able to prove that you developed your stuff independently. Just save all the beta versions, incremental copies, etc. somewhere safe. Remember to timestamp them. E.g. if you’re writing the background story, save a different file after completing each chunk, save the first draft and all subsequent revisions. This should keep you on the safe side.
Or, you may have snagged something copyrighted without checking. In this case, exercise caution when picking stuff from the internets. If you really need it, check for copyrights, contact the author, get permission to use (save all communications if you do). If you can’t find the author or terms of use, don’t use it. Usually open source stuff has it all detailed in some kind of a FAQ - what kinds of disclaimers you need to use to stay kosher.
There are cunts out there who may try to force you into settling out of court, counting that you are too busy for this shit and are easily scared by legal babble. It’s just bluffing, and you can call it simply by asking of proof that they (or whoever they claim to represent) own the materials. It is important that you trust in your work and are able to demonstrate that it’s all yours in case you should defend it at court. Take care not to use “elder” and “scrolls” in the same sentence anywhere and that’ll at least keep the Bethesduh off your ass.
It gets a bit trickier when you work with other people. When you hire artists, make it all clear in written form who is the owner of all the shit (for simplicity’s sake, you should own all the assets made by your team, be sure to clear this up with everybody). If you’re licensing stuff, save all the contracts and arrangements. Make them guarantee that they are developing their shit independently and that shit they produce does not infringe rights of any third parties in any way whatsoever.
It still pays to double-check on your artists from time to time if they aren’t stealing their assets from some website. Easiest way to do it is make them save incremental progress in separate files as you would do yourself. If the 3D artist magically conjures up a hipoly mesh out of nothing in under an hour, you might want to ask some questions.