How To Ask For An Introduction
I have worked on multiple consumer products at Google (mobile maps, mobile gmail, etc.) and Twitter. One of the key things you learn when building a consumer product is to make things as easy, streamlined, and friction free as possible for your users. When asking an angel, advisor, or other person to make an introduction for you, the same rule applies. The structure below saves a lot of pain & back and forth for you, as well as for the person being asked to make an introduction on your behalf.
Bad Introduction Request:
"Subject: Re: Wassup! Mission pub crawl
Can you intro me to Marc Andreessen? Awesome thanks! WOOT!
The issue with this request is a few fold:
1. If Sarah forwards the email as is, Marc Andreessen is likely to either ask her for more details or ignore the request. After all, he is really busy, no context has been given, and there is no explanation as to why Marc would ever want to meet with Julie.
2. Julie is putting all the work on Sarah. If Sarah wants to see the intro through, she will now need to write a bunch of background on behalf of Julie as Julie was too lazy or thoughtless to do it herself. Even worse, Sarah may not have full context on what Julie really wants, or her background. This all decreases the likelihood Sarah either forwards the note, or if she writes it herself that Marc Andreessen replies. Finally, the email subject line has not been tailored for Marc to want to open it (unless he thinks he is being invested to a pub crawl, in which case it might work very well indeed). It looks like Julie just replied to the last email she had with Sarah versus starting a new thread.
Good Introduction Request:
"Subject: Andreessen / Drone & Bitcoin / MIT 
Good to see you yesterday. As discussed I am working on a drone-based bitcoin mining pool. I have pulled together a team of 5 MIT engineers  and I used to run both the bitcoin algorithms and drone design clubs at MIT . I had previously co-founded BTCommunity, the world's largest bitcoin forum .
Given Andreessen's funds investments in Coinbase and Airware, I was hoping to get feedback on our go-to-market model. I may also be able to provide introductions to Marc to other new bitcoin-focused companies as I know most of the founders in the market due to BTCommunity.
Why is this a good email?
It is easy to forward by Sarah without Sarah having to do any work. The subject line is written with an eye towards email open rates. So, this is a low friction email to forward on, as well as low friction for the recipient to open and read.
 Subject line will catch Marc Andreessen's attention and summarize what the email is about. This increases open rate of the email.
 Social proof inserted - Sarah met with Julie live. Since Marc respects Sarah, if she is making time for it maybe it is worth his time too.
 Background and social proof on Julie - Julie is someone worth meeting and an expert in her area.
 Julie actually did research on Marc, and has a real, specific reason to think he might be interested. She did her homework and is not going to waste his time to "just network".
 A specific ask / reason to Marc is stated. This allows Marc to accept or decline based on whether it makes sense to meet and may save Julie time as well (e.g. if Marc does not care about bitcoin anymore).
 It is probably worth Marc's while to meet with Julie with an eye to a longer term relationship. She may actual be a valuable person in his network versus someone who just wants to meet someone famous.
By spending a little bit of time up front you can make life dramatically easier for the person doing you a favor / offering an introduction. It also increases the likelihood dramatically that an introduction will actually occur and yield a follow-on conversation.