We've all heard it, right? Avoid all "to be" verbs. They weaken your writing. Find a stronger verb.
I've used Dickens as a defense. "It was the best of times, it was the worst of times, it was the age of wisdom, it was the age of foolishness, it was the epoch of belief, it was the epoch of incredulity . . ." I've also heard that countered with, "But he wrote like 500 years ago." The person I was talking with wasn't all that bright, I admit.
Okay, here's one that's a little more current. From the best-seller "The Girl Who Played with Fire." We have learned that Lisbeth Salander has a twin sister. A twin sister quite different from Lisbeth, if only twenty minutes younger. Here is what Larsson writes:
"Lisbeth was first. Camilla was beautiful."
Six words, two of them "was," that made me stop and say, "Wow." Didn't need much more than that to tell me volumes about these two.
And another thing about this, Larsson is not only using the dreaded to-be verb, but he's telling not showing. And it's better. He could have gone on for pages, and would have had to, to show us the same thing he tells us in six words.
I read a lot of criticisms about The Millenium Trilogy" that centered on Steig Larsson's story-telling style. That he tells too much, and this keeps the reader at arm's length. I must have short arms, because I've been right there with Blomqvist and Salander through two books, and I made sure I had the third on hand before I finished the second.
Your mileage may vary when it comes to the trilogy. However, the next time you're struggling to get through a passage in a book (whether reading or writing), check to see if maybe that act of showing is what's slowing it down. Perhaps a line or two of telling could get the point across better and let you get on with the rest of the story.