Ask the operator for 7-days advanced purchase deal (it's only available on some destinations, they will let you know).
Ask the operator for companion fares (2nd person gets a free ticket, but must travel at the SAME TIME to the SAME PLACES).
If travelling to US, some States are more expensive than others. For example (happened to me), it costs more to directly buy ticket for New York City then it does to buy one for New Jersey, Pennsylvania. In this case, the bus will stop at New York City, and you can simply get off there, and your bus simply moves to the next stop at New Jersey. What you can do is simply tear off the New Jersey portion of the ticket yourself.
And also student fares like others have already said above.