>>>>>Don't cross chain and you'll be fine.<<<<

What he means by that is, don't click your rear derailleur to "1" and your front derailleur to its highest position (say "7") and vice versa. When the chain is on the large ring in front and the large ring (cog) in the back, you will notice that it is stretched pretty far and at a sharp diagonal. This can speed up chain wear and can also wear down your rings a little faster than they should. It's not that big a deal if you do this when you need to, but try not to make a habit of riding this way and you'll be fine.