Once Dropbox throws you an error that reads “Unable to monitor filesystem. Please run: echo 100000 | sudo tee /proc/sys/fs/inotify/max_user_watches and restart Dropbox to correct the problem.” you’d better adjust settings in a system config file, to keep changes after reboot.
Open system config (as superuser, of course) in your my favorite text editor:
$ sudo vi /etc/sysctl.conf
Add just one line of code to the end of the file:
fs.inotify.max_user_watches = 100000
Save the file (Shift-ZZ, if you are new to Vi).
Reboot computer to apply settings and restart Dropbox.