Which gears combinations shouldn't be used?
This is an old chestnut from the days of 5 speed freewheels and chainrings without shifting ramps. With modern hyperglide cogs and the relatively slinky thin modern chains, cross chaining is simply no longer an issue. While there are very few studies on this, the one I've seen suggests that there is no loss in mechanical efficiency for cross chaining at the angles typical of most bikes. 

In general for a given gear ratio, big/big is more mechanically efficient than 
small/small. So the only science out there suggests that cross chaining on the big
chainwheel is actually more efficient. 

http://www.jhu.edu/news_info/news/home99/aug99/bike.html

Use the gears you like, check your chain wear and replace your chain often and the expensive bits of your drive train will last a long time.