Here is a thread on DK's, the first page describes and instructs you how to setup a pseudo priority based macro DPS spam button.

It works very very well.

http://www.dual-boxing.com/showthread.php?t=33087

Essentially what you will do if you follow the instructions is create one button that FTL assists, IWTs and sends 7 or 8 DPS abilities simultaneously. The way that works is WoW will randomly select one of those abilities to use if they are all sent at the same instant. By putting say, 3 heart strikes, 2 blood strikes, 1 plague strike, and 1 rune strike into those DPS keys, you've created a weighted system where HS is the most likely to be used. It also handles cooldowns exceptionally well. If a key cannot be pressed due to cooldowns/range issues another key is automatically pressed.