I think one of the toughest things about writing sarcasm is that it's not always obvious. It relies heavily on tone, and that's not always easy to do on the page.

You could...
be blunt: "What a great tie," he said sarcastically.
use a simile: "What a great tie." The sarcasm dripped off his lips like vinegar.
use context: He wrinkled his nose and grimaced. "What a great tie," he said.
use the reactions of others: "What a great tie," he said.
"There's no need for sarcasm."
overwrite it: "That's the best tie ever. No better tie in the history of mankind! The mustard stain? Perfection!"

The key is to make it obvious that you intend to be sarcastic. If you can read it the exact same way without the statement being sarcastic, you could confuse your reader.