keep documentation of your work progress, and core files of graphics (.psd's)
meta data is important, timestamps (date of creation, location, etc)
be aware, if people actually do steal your work, and its just a group of teenagers that use them, dont even bother
if legal expenses > real compensation = not worth it
be careful of what youre uploading to graal if you dont know what youre doing
im at a point where i could defend everything ive ever made and have it taken down in no time, but i dont want to damage relationships
its a little bit more complicated with levels, as the original concept of the level is important too, and if theres legitimate documentation that youre the respectful owner of the idea and concept of the level, then you could go against it too
for sounds its another whole story
a very light copyright security measurement is uploading everything you create to an FTP (you must be the first person ever uploading it to the internet, regardless if anyone has seen it or not)
you dont have to neccessarily register a copyright to copyright.gov, unless you want them to handle the legal stuff
you can register patents for certain mechanics and ideas too, thats another story
tl;dr: document everything you do and plan to do very precisely
be very cautious of your choice of words to whoever when it comes to your IP (intellectual property)
even the word "ok" can count as digital signature to permit usage of whatever it relates to