Only been a teacher since I finished uni last year. I'm working casual at the moment because I haven't scored a full time position yet, but I'm hoping to get one for next year (fingers crossed).
As for your sister astro, the main tip I have for her is DON"T DESPAIR! Classroom management is by far the hardest part of the job, and it takes up (directly or indirectly) about 50% of your time. There are no infallible solutions... different strategies work for different kids. Some kids respond to praise, others to a gentle kick up the bum. Some kids needs a friendly approach where you get down to their level, others need strong leadership and guidance. The trick is to find out what every kid needs, and tailor your style to suit them, without losing your own style and identity. If that sounds complicated, it's probably because it is!
The main thing is to stick at it and don't get discouraged. I've found that each day is a little tiny bit easier than the last.