It sounds dumb, but you have to be able to know the guitar with your eyes closed. If you use the Rock Band Stratocaster, there are little braille bumpies that identify the green, yellow, and orange buttons. On the Guitar Hero SG and Les Paul, There is a line on the yellow button. So just make sure you can make your way over to these without thinking about it too much.
Step 2: How to Strum Intelligently
When you strum, you may be tempted to grip that sucker with all your hand strength and wail on it, but a better and more accurate way to strum is to use your thumb on the slow parts, and for fast parts, have your index finger hovering under the strum bar and your thumb over it and just move your arm up and down.
Step 3: Take a Chill Pill
I was recently playing Rock Band with my cousin, we were playing expert bass score duel. I picked Green Grass and High Tides, but I forgot to select bass. Now, my cousin had just begun to play expert bass, so he was nowhere near ready to play expert guitar. And I noticed something as we were playing. Whenever a fast string of notes came, he would wail on the strum bar and frantically fretted on the buttons. So, take this as advice: don't freak out when you play the fast songs on expert. If you get all frazzled from seeing all those colors, maybe you need more practice. Just drop it down to hard and try to get five stars.
Step 4: Rock Out!
Whenever you play, don't be afraid to have some fun! Do some head-banging! Or even just shake your hips or tap your toes. It helps you get in the rocker mood. Develop your own stage show and the ladies will flock to you like ants to sugar! Or not...