I just told 'em I wasn't coming in - never made excuses.
Given that, though, I was Casual, and it was a super market, so I can't really say I'd have cared if they fired me.
One day I went in when I was sick, and half way through the shift, I couldn't take it, so I asked the (new) supervisor if I could leave cos I'd been sick in the morning, and I wasn't getting any better.
Normally, anyone else would have said "Sure, fine", but this evil ** wouldn't have any of it.
She even got the store manager on the phone (who I new personally) to talk to me telling me I can't leave.
I knew he was just playing it by the book, but didn't really care if I did or not, so I tossed my namebadge at the supervisor and walked out.
They didn't fire me, just told me not to do it again.
That supervisor was fired a couple months later after almost everyone complained about her