An easy solution would be to only allow specific IP addresses however this can be hard if your friends are using dynamic IP addresses.
Another way to make FTP a little more secure is to force the FTP server to run through a high port number so people who run port scans don’t tend to run into the high number of ports since these are mostly client ports not server.
Other than that FTP is pretty secure, I haven’t had any break-ins yet but make sure you use a program that logs what is going on so if you suspect someone is doing something naughty you can block their IP.
Also remember to give the right permissions. You don’t want someone accidentally deleting your stuff.
Happy FTPing
