The shorter the better
Some of the best performing subject lines we use are just 3 words long. The objective of the subject line is to get them to open the email, you don't need to explain too much.

Keep it personal
The inclusion of their name of the company name will increase the open rate but make sure its in a conversational format, so no LLC, Ltd or PLC. 

Keep it polite
Don't use uppercase- no one wants to be shouted at. 

Keep it relevant
Have you got the right person in the right department? It doesn't matter how good your product or service is, if its not relevant to your prospect you are both your and their time.

Tell them what you want
Whether its a reply to a question, a meeting or a chance to show off your new product or service, let them know what you you would like to happen.