People often screw this up because they try and get too fancy. I'm sorry, but until you're Apple or Nike you can't get away with a vague 2-3 word tag line that says nothing to your potential customers or investors for a brand you're trying to grow.
Using Apple and Nike as a reference, let's recreate their tag lines to help guide you should be doing.
Apple: Think Different becomes "Minimalistic and beautifully designed personal computers and electronic devices for creative professionals."
Nike: Just Do It becomes "Fashionable and functional activewear and footwear for the modern athlete."
Now here's some bad examples of tag lines of existing companies which I made up. Let's see if you can guess any.
1. Cloud in your pocket.
2. Your home is one click away.
3. Communicate in harmony.
Answers: 1. Dropbox, 2. Airbnb, 3. Slack.
Were you able to guess any? Guess what, I lied. The real answers are: 1. Gmail, 2. Kayak, 3. Skype.
Okay so I didn't lie, but see how the one tag line can be applicable to both sets? That's why you don't want to be vague. The goal in creating a killer tag line is breaking it down into three parts:
1. What you do
2. For who
3. How you do it
So if you have a new mobile app you're launching that's going to compete with Snapchat for example you want to be along the lines of:
1. Securely share self-erasing photos and videos...
2. on your iPhone and Android
3. ...with your friends and family.
Don't overthink it, just spell out what you do for the end-user in layman's terms.