Build everything to be as easy as possible to debug, because everything else you can do at work after a nice cup of coffee, debugging is the only thing you might have to do at 3AM while drunk.

So:

- Set up some kind of logging, by default use rsyslog and set it up so your logs are available somewhere other than the machine where something is running. You will be surprised how often something breaks and takes the server down with it so you can't log in.

- Later on, when you have to debug stuff, READ THE LOGS. I can't explain it, but after years of working on production systems I have noticed that almost nobody actually goes back and just reads the logs when things go wrong. You will find the problem spelled out in the logs 99% of the time. YOU CAN'T JUST GUESS WHAT WENT WRONG.

- Use transactions properly.

- If you use an ORM, do whatever you need to do to keep track of how many actual queries are done. You are going to find that you do some order of magnitude more queries than you thought. Learn how to give the ORM hints to avoid this.