Use sentence case. If the commit is issue related, add a prefix “[Verb #ID] ” (don’t forget the trailing space). Available verbs are Fix, Close, and On (Fix and Close will close the issue).

For example:

[Fix #3] Migrated to AFNetworking 2.0

[On #2] Trying TEAChart

But not:

migrated to afn 2.0

trying TEAChart

Use explicit description.

For example:

Fixed table view cell text overflow

But not:

bug fix

Follow NYTimes Style Guide.

Except that we write method implementation’s beginning bracket in a single line, as:

- (IBAction)dateChanged:(UIDatePicker *)sender
{
    self.dateField.text = [sender.date dateString];
}

One thing to emphasize, DO NOT use any abbreviations unless they are very common. Long variable names are not bad.