Gears: You are doing it wrong

You want to know why your chain is only lasting 400 miles? Of course you don't, because you refuse to listen. I know you want to be all independent and shit, but you do in fact need my help. Otherwise you would not have to replace both your chain and your cassette every time you walk into the shop. Before you accuse the last bike shop of installing a used chain because "...it only lasted 400 miles," perhaps you should pull your head out of your ass, wipe the shit from your ears, and hear me speak thusly: As much as I enjoy myself, I do not talk just to hear my own voice. If you simply do not want to hear what I have to say, don't bother walking in to my shop because :

A) You're gonna hear it and,

B) You're not going to like it.

If you are concerned with chain wear, you will first, keep it lubed, and second, stop cross chaining! Cross chaining is using the big chain ring and the biggest cog or the small chain ring and small cog. Two extremes that put your whole drive train in jeopardy. It's all about chain line. On a single speed bike, chain line is important for several reasons. The basics are described by Sheldon Brown: 

"The word "chain line" refers to how straight the chain runs between the front and rear sprockets. Ideally, both sprockets should be in the same plane, so that there is no side ward motion or stress to the chain. This constitutes "perfect chain line"."

"In the case of derailleur geared bicycles, the chain line is not perfect in most gears. Chain line mismatch can cause the chain to rub against the side of an outer, larger chain ring when engaged with a smaller one, and can cause problems with shifting, especially with the front derailleur. The worse the chain line, the worse the mechanical efficiency of the drive train..."

"Correct" chain line for a derailleur system is a matter of opinion, and depends on the intended use of the bicycle."  (It's a good thing for you that my opinion matters.) 

On the surface of it, cross chaining is about the most inefficient use of gears I can think of. Here's an example: The same customer that complained about chain wear told me that she rides in the "big ring" all the time. That means she is riding in the hardest gear up front. In order to facilitate easier pedaling, she has to use the largest cogs (or easiest gears) in the back. This makes the chain look like this:

(the bottom one ;)

It also forces every single link to have to conform to severe angular contact points 

(about 20 degrees I would guess) every single revolution of the crank. 40 miles at a time, this chain, designed to go in a straight line, begins to break down immediately. This angular contact can be felt in the pedals too. Because of the severe angle, vibration can be felt when the chain is both picked up and let go by the cassette, and chain rings. Plus it can fuck up your derailleur since most road derailleurs have a short cage and are not designed to handle such big gears continually. 

So I tried to reason with her explaining she is not only damaging her components, but "over training" by pushing a harder "lever." This means she is working too hard to achieve her perceived goal. 

Let me elaborate:

She is riding a compact crank set. (Chain rings measuring 50 tooth and 34 tooth)

By riding in the big ring (50) and her largest cog (23) she is pedaling at 57 "gear inches."

That is to say, for every revolution of the crank, she travels 57 inches. 

Should she decide to listen to me, she could shift to her small ring (34) and her 16 tooth cog and find herself pedaling at 56 gear inches, using less effort, and achieving  basically the same result.

And her chain line would look more like this:

This is not rocket surgery here Lady. Really, it is the exact opposite. If my six year old daughter can understand it, you should be able to as well. If you can't, you should just give up. 

Life gets a lot harder than bikes.