This is to tiggerbabe for Beau and also to GlynisT - this is what I did to stop separation anxiety from being a problem with the neighbours when I went out.
Fletcher was rehomed from a back yard breeder aged nearly 4. She had a couple of old x-breeds who lived inside, but all her cockers were outside in the garden - apart from Fletch. It was clear when I got him home why he was not in the garden and why she was rehoming him (apart from the fact he has no interest in females) - he can howl for hours non-stop.
He was my only dog when I rehomed him, but had lived all his life with others. For the first few days, he howled for ages (according to my very nice neighbours).
Instead of doing the 'ignoring stuff when you leave' that books recommend, I did the opposite. I told him I was going to work, looked very sad about it and waved him goodbye, saying 'bye' and waving in a sad voice as I went out - I spent a few minutes doing this. After 3 days, it worked completely, he didn't howl at all (neighbours again!) and it continues to work after 3 years. 3 years on, the waving and say 'bye' works - I don't even have to look so sad!
It's not sorted his separation anxiety, as he still howls if I go out to the car without going through our little routine and he still hates me shutting the bathroom door
, but it has solved all the going out and leaving him problem.
Since rehoming Fletch, I'm had a few other rescue cockers - one in particular was supposed to have severe separation anxiety. They all have got my leaving routine very quickly and all dogs have been OK with being left for a few hours - the one with the 'severe' case' was fine after 1 day!.
I hope this helps. It has really worked for Fletch and he can still howl for England if he's behind the gate and I go to speak to my neighbour in view 3 yards away....., but I can leave him and the other 2 for hours in the house without a howl, or him on his own, alone.