I use professional scheduling software to help me make the playlist which helps ensure every tune in the station library gets played. The selection and order is not all left to chance. A lot of thought goes into the playlist creation and requires a lot of input to get it right.

Votes made by listeners also have an impact, providing there are enough that is i.e. if a track receives a lot of low votes I may move it into a slower rotation or remove it and likewise, if a track receives a lot of high votes it may move into a higher rotation and play more frequently for a period of time. So remember to vote if you enjoy a track...

I don't tend to have hard and fast rules but instead tend to do it by feel and I will move tracks around to keep it all sounding fresh and enjoyable.