By the way, if DS2 stays awake for more than two and half hours, he can get completely hysterical and will cry loads and will take ages to go to sleep. The idea is to try to 'predict' when he will be tired and put him down before he becomes overtired.
Some people have recommended the baby whisperer book, it is very good I think.
What the author suggests is that you write a 'diary' of your baby's sleep time and feeding time , and see if there is a 'patern' so that you can predict what they will want next.
In my experience (limited, mother of two) if a baby is crying loads and he is fed and changed, sleep is the next problem down the list. You can put him in his cot and leave him for a minute or two, he might just settle down and fall asleep. Sometimes they just get worked up and overstimulated and need to be left alone, not for long though, I'm not talking about control crying just a bit of 'time out'.
** with both my boys, the sound of appliances such as a washing machine, tumble dryer, even hair dryer helps settled them down if they are crying. I know, it's silly but it really calms them down!