Sometimes the most helpful solutions are the easiest.
For anyone who has ever shot with an old, low-quality, or extra long zoom lens, you might be familiar with lens creep. This happens when the lens barrel extends, or creeps, due to its own weight when it's pointed up or down. It's incredibly annoying and can actually cost you big time when you need to get a particular shot for a project. There are several inexpensive items out there that help stop lens creep, including the Lensband which is specifically made for it, but if you want to keep your zoom lens in place and not spend a penny doing it, this video by Dr. Jake Newman offers a free solution.
Ah, good ol' rubber bands—always saving filmmakers when they're in a pinch. As Newman points out, this idea seems almost too simple to share, but to be honest, I've never thought to do this before. In my early days, I had a few temperamental kit zooms that gave me the business every now and again, and like a true noob I had no idea what to do other than keep my hand firmly against the lens barrel to keep it in place.
So, slap a rubber band on your lens to keep it from creepin', or if you want your lens to look a little more stylish, Lensbands are $7.49, work for DSLR and M4/3 lenses, and come in a bunch of different colors—might be worth a look.