TT? *tut* - you dont need him
Changing gear - OK lets see if i can help
You should really be changing gears when the bike is producing enough torque to give you the most efficient acceleration, basically when the engine "wants" a gear change.
Timing is the hardest thing to learn when changing gear.
You should be aiming to select a gear when the engine is running in a decent rpm range, which is usually the bottom of the powerband. You should only change when you need to change - changing to early will lead to insufficient acceleration which in turn could lead you to stall your bike. Too much and the engine will scream blue murder at you.
You need to listen to your engine and get used to how it sounds. You say you might be changing too early - try leaving it a little later, see how the engine responds then. Its something you learn about your bike and not something you can do by reading a book or being told - you have to know your bike. However, here's a little rough guide that might help you - every bike will be different, but here are some basic guidelines for when to change up when you accelerate and change down when you are slowing down to keep the engine spinning in the desired rpm range:
Changing points:
First Gear: 0-10 mph change up at 10 mph
Second Gear: 5-25 mph change up at 25 mph, change down at 5 mph
Third Gear: 20-40 mph, change up at 40 mph, change down at 20 mph
Fourth Gear: 35-55 mph, change up at 55 mph, change down at 35 mph
Fifth Gear: 55+ mph, change down at 50
HTH