Assuming you have the height needed to meet todays code you could use the same thing the power company uses.
Multiplex cables utilizing a bare conductor, factory assembled and twisted with one or more insulated conductors, such as duplex, triplex, or quadruplex type of construction.
Or UFO cable supported by a messenger wire.
Take a look at NEC Article 396.
edit; Thats UF cable
UFO cable is tough to find.
[This message has been edited by iwire (edited 09-27-2004).]