The key to this is not use macro's, you are better off practicing manual casting, and not forcing a rotation. If you set a rigid rotation you will never hit your max dps.
The standard rotation I start with is:
sw:p vt mb mf:3 sw:p -- this will setup your shadow weaving
There is argument over useing sw:p first but the general consesus is an instant cast at the beginning to boost your hit is a loss in time worth having after all shad priests are best at long term fights. Make sure you refresh it after you hit the 5 count though such that it is maxing it's dps potential.
Next I cast:
DP, sw:d
after that you probably have a mf:1 or 2 and hit mb again.
Now all you do is keep up the sequence of 5, cast mb and sw:d whenever they are up, cast dp and vt when they are about to wear off your mob (vt will go first and dp after always in that order if you follow this sequence at least with tier 9 gear that has been the case) and buffer with mf.
keep in mind sw:d, dp, and sw:p are instant cast, be ready to hit another spell almost imediatelly after because your global cool down is the only pause you have to wait for. If sw:p ever wears off it becomes your priority, that spell probbly has the best dps turn around.
I am not a fan of addons but there is a simple graphic one called "shadow priest green light", that is really helpful when it comes do maintaining your shadow weaving. It also helps flag additional spells that could benefit your sw:p and tells you to recast.
On trash you will probably only get the spell line out once, past that you have to judge if it is worth recasting vt or just finishing them off with mind flay, also keep in mind you will want to rotate your shadow pet through, if you have vieled shadows you should be able to run him out 2-3 times per boss fight so use him as soon as he is up.
