I take the front wheel off and slide sections of inner tube over the fork legs to protect the paint. Then I turn the fork around so that it faces the rear of the bike. Next I push the fork legs down through the my rear rack. I secure the bike with a couple of Coghlan's Arno Straps and use some toe-straps to secure the front wheel to the bike being towed. With this method the towed bike is extremely secure and I don't even know it's there.
Here's an image of a Miyata 710 I brought home along with my wheel truing stand. It was one time when I didn't use the inner tube segments on the fork legs.
Or you can buy a front fork mount similar to these.
Cheers