Have a buddy
If at all possible travel with a friend. This has multiple benefits including being able to sleep on their shoulder and having someone watch your stuff while you run off the bus to pee on the side of the road (African buses of course). Now if you are like me and often travel solo this is a real treat to have company on a long bus journey.
Have a good book.
Anything that will grip you and make you feel like minutes have passed instead of hours. I find my kindle invaluable in this instance as, if I am finding a book is dragging and not gripping me, I can change and make the hours fly by. (Note that in Argentina and Chile and South Africa they show movies on board some of the buses but sometimes they are not the type you would want to watch, think strong Christian beliefs in movies shown on the Intercape buses)
Always take snacks. Argentinian and Chilean buses tend to give you meals on longer journeys (I got a hot dinner with wine and champage after dinner on my 18 hour bus from Buenos Aires to Iguazu Falls) but must buses don't. Often the stops are at fuel stations in the middle of no where with very limited (and expensive options). I usually take an apple, some nuts, a sandwich and a cereal bar with me and a BIG bottle of water. Never underestimate how dehydrating travel can be.
Make yourself comfortable
On long journeys comfort is important. If you are taking an overnight journey, you don't want to waste the whole day that you arrive being tired and cranky and just wanting to sleep! So I always bring a neck pillow (I have an inflatable one which takes up about as much room as some tissues), an eye mask (nothing worse than being woken up by someone's light/ an very early sunrise) and a blanket (when aircon does work, it can be super cold). I always try brush my teeth at the last stop before I think I'll fall asleep as it really puts me
into sleepy mode.
Dress not to impress
Comfort is key, not just in what you bring with you but in how you dress. Comfy clothes are required so elasticated waistbands are your answer, think yoga pants, tracksuit bottoms, cotton t-shirts. Always bring a cardigan (A/C again) and socks!
Late night the night before
This is a controversial one but if you are taking an over night bus that is not one of the amazing Argentinian ones, I would stay awake as much as possible the night before so that you are more likely to fall asleep the following night!
Let your imagination run wild
Am I the only one who makes up stories in my head on long journeys or while walking long distances? Yes. Oh well. I love to let my imagination run wild and think up stories involving the people around me or people I know who are far away or just how my day will turn out (this usually involves being whisked away to an amazing restaurant all expenses paid- yet to happen!)
At the end of the day any bus journey is worth it because when you get off, you are in a new place and get to explore. (Someone remind me of that when I'm on my next long journey)