Double wrap belts are the way to go for earlier swords. They're more comfortable, don't sag, and they're period as well. I have this one:
http://www.museumreplicas.com/p-145-medieval-double-wrap-belt.aspxand it works well for earlier swords (under 40" or so). It's a roughly 12th/13th century belt style, so it's appropriate for the type of swords that were most common then. If you have a longsword or other bigger sword, a 14th/later style belt works best. I plan to get the "swordbelt for long sword" at the bottom of this page:
http://www.theinnerbailey.com/swordbelts.htmIt is not a double wrap belt, but the distributed strapping on the side and rear lets the sword sit at a proper angle and spreads the weight from pulling down on one spot and sagging.
Pardon the lack of mail, the pirate boots, and the spaulder sitting under my breastplate during the pic, but here's the MRL double-wrap sword belt "in action":
The sword is a CAS/Hanwei Great Sword of War, and is almost 48" long. It kept bumping the ground (I'm about 5'11"), and I think a max sword length of around 40" would be best (leaving ~6" of ground clearance).