Hold the guitar correctly. If you have the guitar upside down or back to front this will blow your cover. With a right-handed guitar, the neck of the instrument should point to the left. The strap should go over your left shoulder and around your back so that your left shoulder bears the weight. Don't put the strap over both shoulders. If it is a left-handed guitar, do the reverse. If you are sitting down with the guitar, don't use the strap. Rest a right-handed guitar on your left knee and vice versa.
Move to the rhythm. If you are miming to a song, you should move in time with that song. Listen to the song and count along with the beat. Guitar players will naturally move their bodies as they play along, either without knowing it or as part of their performance. Make your movements subtle and natural. Nod your head and tap your foot in time with the music.
Position your left hand correctly. While you may not be playing the exact note, if your hand is roughly in the right place people will rarely notice. High notes are played on the thin strings and low notes are played on the thicker strings. Really high notes, like the ones you hear in guitar solos, are played near the top of the fretboard. Remember that the top end of the fretboard is nearest to the guitar's body. The low end of the fretboard is near the guitar's head. Move your hands whenever your hear a noticeable change in pitch. Chord changes normally happen in counts of four, so count in fours as you pretend to play to make sure your hand changes position at the right time.
Strum convincingly. Hold a plectrum between your thumb and first finger. Move it up and down across the strings in a controlled manner. All movement should come from your wrist, not you elbow.
Talk the talk. If you really want to appear professional, use guitarist lingo. Call the plectrum a pick, say "axe" instead of "guitar" and ask how many bars you get for your first solo. Make sure you look like you're having fun.