Our drive train can handle whatever gear combination we plug in right? WRONG! It is imperative that we know how to work with our drive train to prevent unnecessary wear and expensive damage; not to mention performance advantages.
Don't Cross Your Chain
Your chain has been crossed when you are in the big chainring at the front and a big chainring at the back (eg. 3x1), or the small chainring at the front and a small chainring at the back (eg. 1x9). Riding in these sort of gear ratios puts extra strain on your chain which can lead to premature chain/cog wear and most likely a snapped chain.
When shifting you should always be wary of the ratio you are in. When you are in the middle ring upfront you can use most of the range in your rear cassette but your drive train may struggle with the smallest and biggest few cogs at the rear. When in the smallest (easiest) chainring upfront you don't want to exceed the inner half of the rear cassette which contains the larger (easier) cogs. The same principle applies when in the big (hardest) chainring upfront - stick to the smaller (harder) gears at the back. The straighter the line of your chain, the less strain it is under.