I need to improve my process, but what I do at the moment is use Dropbox like you But I also have a couple external harddrives that I just copy all the files from my desktop to every month.
One hard drive is under my bed, the other is in the cupboard at work so that it's not in the same place as the computer. The one at work I bring home and update from time to time with new files
My issues are 1. trying to keep the folder structure on my pc nice and tidy so it's easier to backup and also 2. taking a few minutes to see how to update the folders on the external harddrive so it only updates the new files, I normally just copy the lot even if it already exists but it takes ages and I'm sure it's possible to do it quicker I'm just concerned it will miss something.
I used Crashplan for a few years too, which is an app (on the PC, iPhone, Android etc) that runs in the background and constantly backs up everything while you work, it can also keep external hard drive backups too. It was only about £5 a month and worked really well, but when I got a new PC I cancelled it as it does slow the pc down a little; it was cool though.