How do I hide my tell when I am bluffing?

Smirk when you have a bad hand too. Every hand, bad or good, smirk.

Become the Joker.

Practice, practice, practice.

Stop smirking?

Bite your tongue or chew on the insides of your cheek

Never break eye contact with the person your bluffing to and keep your body language casual and yes practice.

I have to go with @Sunny2 on this one. Once you break the association between your reaction and the quality of your hand, reading your bluffs becomes impossible.

Many try to be stone-faced and give no displays of even being alive, but that makes even the subtlest twitch noticeable. If you grin like an idiot no matter what cards you have in your hand, then any other player with even the slightest hint of pattern recognition will learn that your smirk could be anything from nada to Royal Flush.

maintain regular eye contact, dont smile, and immediately follow up with a question to their response.