The importance of the pitch cannot be understated. I am no master at it, but I can relate what has worked for me over time.
When I first started, the content of the pitch was mostly about the mechanics which I thought were cool and my motivation for doing the game. This was long winded and could be rather boring, well for some. Also it was really just me saying things. I didn't have a reference to point to.
In my case the biggest grab is the art. When I pull out the sell sheet, I can see it in peoples eyes, the world kind of creates this warp field around their brain as it sinks in. It is really cool to watch.
Also what I talk about has changed, it is no longer just the mechanical stuff, it starts with the narrative. Almost like the back of the box. Between that and the sell sheet if people are interested I go into a second pass which is some key mechanics that make my game unique.
Different people latch onto different things, but I find most like the narrative first, and closing with the unique mechanics hook those that are into that part. The most important part is you want to leave them wanting more.
"Leave them wanting more"
I can get through the pitch in about a minute or two. Maybe this is too short, but for me it is enough for both parties to gage interest. For the most part I can tell by the end if they are into it or not by reading their body language.
If they aren't into it, I don't want to waste anyones time. If I am not sure, I ask if they have any questions. I only do this if I am unsure because some people are too nice and come up with something just to pretend like they are into it. If they are into, they usually just ask questions right away.
Some people are slower to consume the information. In that case I just let them know to ask me anything if they have questions, then excuse myself if there are other people around to help. This seems to work well because they pour over the sell sheet and usually come back if they are truly interested.
Where I Use It
I have given this pitch to; publishers, a company I was looking at licensing their brand, press, general public, friends, and family. I have pitched at two conventions, over video chat, at local game stores, in peoples houses. Hopefully I will get to pitch in an elevator at some point.
Basically this should be the first thing you say about your game to just about anyone and everyone.
Be Ready for the "More"
After you are done with your pitch be ready to follow it up with more information from either their questions, or you leading based on their reactions to the different parts of the pitch. Again, be reading body language to make sure you haven't stepped off the path of engagement.
So here is my pitch. I start by handing them my sell sheet. If they are really into it I might just go with Q&A, but I really try to get through this:
"Imagine you have woken up this morning and you are now a robot," as I point to the character image on the sell sheet.
"But something is wrong, your pet robit is not around," again pointing to a robit on the sell sheet.
"Your quest is to try and find all the missing robits with your friends. Together you tell stories about your adventures. In the end, will you find your missing robit, will you solve the riddle?"
This is where I stop and see if they are engaged. If so I might then go on with the mechanics.
"In this intro to role playing/story telling games for families and those wanting a quick filler, you use a one of four choose your own adventure style game books as the meat of the story."
I don't go into more detail unless they start asking questions.
Practice your pitch, change it as you get feedback, make it work for you and your product. It should leave people who are truly interested wanting more.