If they live 4 hours away and there is a place two hours away from both of you that you both like to visit then meet there to do the exchange.

I agree that meeting to exchange them is the best option but if it isn't an option here's some tips.

Make sure the temperature is good. Don't ever try and ship worms if the temperature is too hot or too cold for the worms to survive in.
Put them in a cloth bag or a brown paper bag will work well also.
Make sure that they spend no more than 2 - 3 days in the mail system. ( The longer someone else is handling your worms the more of a chance they won't arrive in good shape)
Make sure the bedding material you use to ship your worms in is very dry. When shipping worms moist bedding will be very bad for air flow and will likely kill worms. So make sure the bedding is quite dry.

That's all I can really think of right now...
Happy worming

Thanks for the tips, equinoxequinox and billthewormman. I'm thinking of sending them via Greyhound.

Worms on a Bus