Vanef/STC (formerly the State Transport Corporation) and Ghana Private Road Transport Union (GPRTU) buses are available for long journeys (for example Accra to Kumasi or Bolgatanga) at a very reasonable fee. They have stations in most major cities -just ask- and the routes are usually "express" i.e. they don't make many stops.
One can prepurchase tickets on all the routes of Vanef/STC, and it's worth asking for the "luxury" bus which is air-conditioned usually only a few cedis more. Although all seats are supposed to be reserved on both bus lines, don't count on your seat being reserved unless you obtain advanced tickets. Get there early if you are not holding advance tickets, because ticket lines are invariably long. Schedules, unfortunately, are not followed and it's not unusual to wait for an hour or longer for your bus to leave.
The concept of a "rest stop" is somewhat new to Ghana; consequently, there are very few "comfortable" rest stops along the road. The Linda Dorr rest stop is the exception; well-lit, clean, with a restaurant, small shop and even a (for 20p fee) rest room.
It's important to note that in recent years, there has been an increase in armed robbery against bus passengers, especially during the overnight hours. For that reason, it is best that you travel only during the day, and if your trip may extend into the evening, consider stopping and staying at a hotel before continuing the journey the next day.