I’m sure most of you read wow.com already, but if you don’t, please check out Allison’s latest Shifting Perspectives column on troubleshooting cat DPS. Some excellent advice there (of course, the linkback doesn’t hurt).
Failure to energy pool. If all of your buffs/DoTs are up, it’s tempting to spam Shred as soon as you have the energy required. Don’t. Let that energy accumulate up to about 80 or so before firing off that Shred; this gives you a small reserve for when you DON’T want to wait (Rip down and need CP’s, etc.)