I first took my DS to sleep school when he was 4mo (for a day stay). and again when he was 6mo (for a week stay).

Consistancy will help. Ideally his cot, in a darkened room, and going through the same procedure each time.
The operation would have unsettled him. So does things like teething or illness. It's frustrating when you get a great sleeping pattern - then a tooth raises it's ugly head and throws it all into turmoil!

And as you said... putting them into their cot while they are awake is also something they recommend. If they can go down awake, then fall asleep - this helps them to resettle themselves if they wake up later on.

Good luck with it all.