Seek and Destroy "was"
I've left this one to last because when you search your manuscript for 'was', prepare to be there for a while. And each replace will be a little more involved. In most cases you won't be able to simply delete 'was' because you'll have to change the tense of words around it. But the seemingly endless task is worth it.
If your writing is infected with pests, you know it. You can't let your favorite pest bother you while you're writing first draft material, nor while you're revising, because you'll block.
However, once a draft's pretty much the way you want it, it's time for pest control. I try to eliminate most "was" constructions.
The bright side: once you recognize your own pest, you can instigate pest control at will. It's fun to murder the little beasts. :-)