I know that it is a controversial subject, but we used "controlled crying". I actually think that's not a great name for it as our baby cries less now than before, and we didn't really control his crying - we allowed him to teach himself how to go to sleep on his own.
We waited till he was 6.5 mths old and we tried everything else first - he was on solids, had his own room (was with us for first 6 months), long naps and short naps in day, etc etc - but his sleeping was just getting worse and worse.
He was waking up every 2 or 3 hours and crying till I went in and shushed or fed him. I felt terrible for him as well as, obviously, for me, as I was completely knackered.
Controlled crying - it took 25 mins the first night. I went in after 1 min, then 3, then 5, then was going to wait 8 but he was asleep before the 8 mins were up (I stayed him his room for a few minutes each time, not picking him up, just patting him, telling him he was OK, we loved him, etc). He then slept till 7,30am that night and ever since has slept all through - just a couple of times he's cried again and we've done the same thing. So, less crying overall and I am happy that my baby can sleep by himself - he is happy about going to bed now, and actually when I'm kissing him goodnight in my arms, he wriggles these days because he wants to get in his cot.
Again, I know it isn't or everyone, and I actually was convinced I would never do this - plus had to psych myself up massively - plus every minute was physical agony - but I'm posting this because I think it was worh it for us, and I don't think you'll get that many other Mumsnetters saying so, because, for whatever reason (and I love t his site, so this isn't knocking it) MN seems to be a more continuum, BLW type place.