Things I have learned so far during my short time as a bike mechanic:

While these lessons come from the bike shop, many can be applied to life in general...

Go slow

Remember what order things go in when you are taking them apart

Try simple solutions before more difficult ones

Size matters

Be careful

If it doesn't feel right, it probably isn't

If you spend more time diagnosing problems at the start, you'll spend less time re-doing things later

Have fun

Relax, it's not that deep

Strive for excellence

Ask for help when you need it