Email Subject Lines

It’s well known fact that the subject line in email determines the open rate what word or phrase have you seen to work best?

I have had good success using
Free and Exclusive

Also do you prefer HTML emails or just plain text?

We always go plain text

Usually something like

RE:Window Cleaning

Or

RE:Pressure Washing

Also

“Sorry…”
“Introduction”

[MENTION=3164]Brian_C[/MENTION]

I like a mix of plain text and html

BRAND NEW
Hey Brian FYI
Brian, important
Brian, thanks