No solutions from me, I'm afraid, but sympathies to you all - DS is 11 weeks, needs to sleep most of the time and seriously struggles to get to sleep.
He used never to go to sleep without crying, because he was chronically overtired. However, he has caught up on a backlog of sleep and is generally less overtired now as I've been wearing him in a sling for daytime naps - ie pretty much all day every day. Yes, it's doing my back in, but he is better than he was. DH takes the sling whenever he's home, and my dad has kindly visited to do some of the babywearing. I have this sling which I find fairly comfy.
Nearly 2 weeks into using the sling, he cries when we're getting him to sleep in the day, but not at night - he seems more relaxed. He cries less when going to sleep in the day than he used to. However, it's not been a miracle solution, and I doubt it would work for everyone.
I agree with lazaaroo about spotting the 'tired window' - I'm not that great at it, but it definitely makes a difference with DS if you miss it. We were never able to identify it until DS had caught up on sleep after being in the sling a few days - only then did he start yawning, looking tired etc.
I have friends who had babies like this. They keep telling me it does get better. .
Chin up, ladies - as DH says, every day gone by is another day less to put up with this, and another day closer to things improving both for us and for our little ones.