Have you ever been unsure of a telephone number? You know the numbers, the digits, but you often get two of the numbers round the wrong way. When you do, you don't get through!
The Sales Presentation is exactly the same. It has a rhythym and a sequence. Get it in the right order and you succeed. Get it in the wrong order and you are out on your ear.
When do I close? Well I'm one of those people who hates being sold to (and hates selling) so I tend to close at the client's initiative (only). In other words, I try not to take the initiative in closing or (more reasonably) I only take it when it's screamingly obvious that the client has already said 'yes' in their heads.
To me, if I have to make a "formal" effort to close, I've screwed up the rest of the presentation: the client's needs should drive the agenda so I don't need to do the horrible "sales" bit at the end.