What a brilliant idea. I've no idea how we've survived up until this point without this marvellous invention.
Seriously. That's what careful usage of italics is for. Sarcasm isn't supposed to be blatantly obvious. This will be like in real-life conversation saying something and then immediately after saying "That was me being sarcastic, by the way."
If you're doing it properly and the person you are talking to isn't a complete moron then sarcasm shouldn't need to be highlighted like this. It's pointless.