Key issues have been covered above, but to summarise:
* Decent length tow line secured to mast
and primaries or other very strong and secure points with chafe protection. Mid point floats also help too see what is happening
* Both boats manned and actively steered
* Communication important to convey your intent
* Lead boat is in charge
* Towed boat steers for transom of towing boat.
* When you tack, the towed boat gives enough slack to allow the lead boat to establish way
* Finally, obey the rules, use the shapes and lights appropriate but still keep the spotlight and white flare handy. Idiots abound and I do not like gathering yet more on the tow line.