I saw a good chart recently about this. Wish I could remember where. But it showed the whole "don't cross the gears" thing very clearly. Basically (for a 7 speed):

• Lowest Chainring: gears 1-3
• Middle Chainring: All gears
• Big Chainring: 5-7

Since I saw that, I mainly just leave it on the middle ring and shift the rear deraileur. On a big uphill, when I drop down to 3 or 2, my next shift down will be to shift to the low chainring; on a big downhill, I'll go up to 5 or 6, and then kick it up to Big. With a little practice it's become second nature.