We've only really been able to do it three or four times in our 5 month old DS's life, all in the last two or three weeks. When its happened its been an absolute delight, but it seems a rarity. We must have caught him when he was in a particularly good mood, but every time its been something different (raspberries on his neck, playing peep oh, and talking to him in a funny voice saying boo boo boo boo... 
The rest of the time, he's an extremely contented baby and so easy to make smile. But other babies his age seem to laugh loads! Has anybody got any suggestions?