1. Separate subject from body with a blank line
2. Limit the subject line to 50 characters
3. Capitalize the subject line
4. Do not end the subject line with a period
5. Use the imperative mood in the subject line
6. Wrap the body at 72 characters
7. Use the body to explain what and why vs. how

A properly formed git commit subject line should always be able to complete the following sentence:

If applied, this commit will <<your subject line here>>

For example:

• If applied, this commit will Refactor subsystem X for readability
• If applied, this commit will Update getting started documentation
• If applied, this commit will Remove deprecated methods
• If applied, this commit will Release version 1.0.0
• If applied, this commit will Merge pull request #123 from user/branch

Notice how this doesn't work for the other non-imperative forms:

• If applied, this commit will fixed bug with Y
• If applied, this commit will changing behavior of X
• If applied, this commit will more fixes for broken stuff
• If applied, this commit will sweet new API methods

Remember: Use of the imperative is important only in the subject line. You can relax this restriction when you're writing the body.

Reference: http://chris.beams.io/posts/git-commit