He has never really been able to. If we leave him to cry he becomes hysterical. If we comfort and then leave he just cries again. We usually spend about 1/2 an hour going in and out, then finally give up, calm him down and then pat him off to sleep. This takes up to an hour at nap time and another hour at bedtime.
Once he is asleep he is usually fine (although he didn't learn to sleep through until about 13 months)
I have tried dropping his day nap, which works to an extent (he falls asleep far more easily at night) but means he is horrid all afternoon and falls asleep at 5.30 / 6pm, which means an early morning.
We have moved him into his sister's room, but it doesn't seem to have made any difference.
Maybe its just one of those things we can't change - if so, stories of children who eventually learned to put themselves to sleep would be much appreciated!