As I said in a thread just a moment ago, I used a somewhat unconventional approach around your level.
I elected to use PW:Shield such that I did not have to concern myself with Mind Flay interrupts or fearing mobs away from me. My sequence was as follows:
PW:Shield --> Mind Blast --> SW:Pain --> Mind Flay to 25% (usually two casts) --> Wand
Depending on how much spirit you have, you may be able to endlessly use this sequence with no downtime other than letting your Spirit Tap finish. I recommend checking the AH for cheap "of spirit" and "of the whale" gear to replace anything you have that is massively outdated. I'm usually not one to advocate buying off of the AH, but these two types of gear are usually very cheap, as the shadow priest is basically the only spec that gets much utility out of spirit in the mid-levels.
Like I said, my approach is generally regarded as unconventional. However, in the two priests that I've capped, I used some variation of the above rotation, and I only had to eat/drink after multi-pulls.