I always use a flow-chart mentality: Is this the initial email or response? use greetings and salutations. Is this a reply (ie, email 3 or 4 in the chain), only use their name and comma (and only sign with your name). If you're acquainted or coworkers with the person, you could probably drop salutations, but I'll still use "Hi Person" (don't use "hey" though unless you're friends) and "Thanks, Sol". If I say thanks in the email and concluding the conversation, I use "sincerely" or drop regardless of familiarity. I think that's 90% of all emails I send.
But things get complicated when you're trying to get a point across in that ever so delicate corporate-speak. I also don't really get it, but I've been told a very well-crafted sign-off can actually send a message (one time I had my dad help me craft an email asking for something big, he was describing something not unlike jedi mind tricks with something simple as "Your assistance is greatly appreciated". It totally worked.)
My question: how the heck do you pronounce Amish? Someone told me I was stressing the a wrong but I literally can't tell the difference.