Practice your viola every day to become a virtuoso. Bravado playing requires the ability to know that every time you perform you will play perfectly. This requires daily practicing of scales, arpeggios, tone development and bowing technique.
Perform as often as possible in front of friends and family, outside grocery stores and even in the park. Everyone gets nervous when playing in front of people, but the true bravado virtuoso knows that he must never show his anxiety to the audience.
Play the music with your own interpretation. People don't come to hear you perform another violist's interpretation of the song. They come to hear you, so give them your own interpretation. Playing a piece at an unusual tempo, using extra portamento or changing the music to play in a different register are all ways to add bravado to the music. Play with a technique that will allow the audience to remember what makes your performance distinctive.
Talk with the audience and give them some musical history or an interesting fact about each piece or group of pieces before performing. This will help you connect with your listeners and separate you from the performer that only plays.