I think a little sarcasm goes a long way. Don't overdo it. Even with a very sarcastic character.

Second, I usually make sure that the sarcasm is easily recognizable as sarcasm without using an adverb in the tag like: she said sarcastically. A lot of times, I do this by having having another character recognize it as sarcasm. Something like:

"That went really well," she said.
"Okay, I really don't need your sass right now," he said.