Security is very important for a VPN. That's why you're building one in the first place, isn't it? You need to keep a few things in mind while setting up your server.
Since this server is going to be on both sides of your firewall, and set up to forward traffic into your network, it's a good idea to secure the box as well as you possibly can. You can read up more on Linux security in the Linux Security HOWTO. In this case I killed everything but sshd and a Roxen Web server. I use the web server to download a couple of files (my scripts, etc) for setting up new machines to access the VPN. I don't use an FTP server since it's harder to configure one to be secure than it is to just make a few files available with a web server. Plus, I only need to be able to download files. If you really want to run different servers on your gateway, you might want to think about restricting access to them to only those machines on your private network.
Yes, it sounds kind of silly, but it got your attention, didn't it? No, you don't use passwords, you disable them completely. All authentication on this machine should be done via ssh's public key authentication system. This way, only those with keys can get in, and it's pretty much impossible to remember a binary key that's 530 characters long.
So how do you do that? It requires editing the /etc/passwd file. The second field contains either the password hash, or alternatively 'x' telling the authentication system to look in the /etc/shadow file. What you do is change that field to read "*" instead. This tells the authentication system that there is no password, and that none should be allowed.
... nobody:x:65534:100:nobody:/dev/null: mwilson:x:1000:100:Matthew Wilson,,,:/home/mwilson:/bin/bash joe:*:504:101:Joe Mode (home),,,:/home/vpn-users:/usr/sbin/pppd bill:*:504:101:Bill Smith (home),,,:/home/vpn-users:/usr/sbin/pppd frank:*:504:101:Frank Jones (home),,,:/home/vpn-users:/usr/sbin/pppd ...
Note that I've done more than just editing the second field. I'll explain the other fields later on.