Our B is about 5 months old and he has moderate separation anxiety. We started using the "peekaboo" training with him about two weeks ago and he has gotten a lot better. We started by being in the same room with him and just ignoring him (not touching him, not looking at him…) then we move into another room where he could still see us, but not be right beside us, next we went into a room where he couldn't see us anymore but could still hear us, and finally we left for short periods of time. He has been good enough to leave by himself, uncrated, for an hour or two. I'm not sure about longer periods of time yet, but we will work on it. They are just SO smart and he is starting to realize that we are coming back for him.
We tried crate training him but it seemed to be a useless goal. In a short period of time in the crate, he beat his head off the crate so hard that the left side of his face was swollen so much he couldn't open his eye We still kept trying and the last straw was when he was left in the crate for about 45 minutes and he broke off the metal bars of the crate. I am more scared that he is going to hurt himself in the crate than out.
Since he has been out of the crate while we are gone, the worst he does is finds tissues and chews on them.