Startup are for sale
What's most unique about startups, in my opinion anyway, is that the end game of the business is to be bought out. At least in the current fad.
That puts an interesting spin on what has to happen with the code. Best practices tell us about extendable, secure, well written, gracefully degraded / progressively enhanced code. The startup owner probably couldn't give two hoots, so long as it LOOKS like it works, and is compatible with the investor/buyer's browser.
This presents a conflict of interest, the largest part of my struggle with startups - I want to write code as it is supposed to be written. Excuse the pretentiousness, but I think it is an art, and I enjoy it as such. Take out the art, I stop enjoying my work, and write even worse code.
So what can be done to curb this attitude? Well, an employee probably isn't going to change the goal of the startup, What we CAN do is change the goal of our code.
I'm writing extendable code so that I can use it in my next project. I'm effectively writing a library of code which can be easily configured to work for the next startup and then the next. This is how frameworks get started. You can even contribute back to the open source projects you used to make your code work - and don't feel like you are going behind your employer's back - they get the benefit of your code from the last place you worked, and from the open source projects they didn't pay for.
It's all good, just play along with the cycle.