You can find tons of blog posts about cold emails with different techniques, etc. For me, I had some key takeaways. (FYI, I don't have any hard data to back my claims, only my experience and feelings.)

- If the market sees value in the problems or solutions that you're pitching, the market will respond.

- When I start my emails being open and giving about myself and background, I seem to get more responses. I usually open up with "Hi, my name is Minto. I'm a software engineer in Austin, TX, developing X."

- I'm very specific about the person's niche. ie. "I'm developing X for <Y niche>."

- My emails are very laid back and unpolished. I try to come off as informal as possible, as if I was writing to an old friend.

- I approach from a place of learning instead of selling. I think people, myself included, don't like being sold to, but like to help a fellow human being out. The fun and exciting thing is that if you hit on the interview with the right problems and solutions, you've pretty much sold them.

- I put a timeframe on my ask, "30 minute coffee", etc.

- Adding your social identity seems to help. I add a link to my LinkedIn profile in my signature. I can only imagine that this adds credibility and many people I email will ask to connect with me on LinkedIn.

- Referrals get high responses. Not really a cold email, but the technique of having a subject line of "Referred by X", gets a high response rate. I only do this if I was really given a referral by someone.

- Be short and sweet. I'm not a very verbose person as it is, but I think people will tend to read your emails if they're short.

I haven't done any cold calls yet, when I do, I would approach it in a similar way. I am pleasantly surprised at the responses I've gotten and the willingness of people to help.