So I've now been developing large scale web applications for a few years now, and I thought i'd share a few points I believe every coder needs to think about.

If you're just starting out you wont know why you NEED to do these things, but anybody who has had to re-visit there own or someody elses poor code will soon tell you that it's far better to get it right and scaleable from the start than to have to go over everything and try and change it 6 months down the line.

So, in no particular order..

1) Don't rush to start

Think things through before you write it. This goes hand in hand with point 10, if you think something through to begin with and do it right the first time, it'll be easy to expand upon and improve later.
If you rush straight in you'll get half way through, get stuck and have to start all over again.

2) Plan to scale the codebase

Many problems in developing applications come from having "organic" code, we've all seen it, code that just grows and grows into a behemoth of unsupportability.
All projects, whether big or small need to be factored into well organised, scalable applications, From! The! Start!
No silly naming of functions, having a function "set()". It needs to be "set_foo()" at a mimumum so that you can have "set_bar()" as well, having just "set()" is going to get confusing VERY quickly, don't do it!
Consider using an MVC like framework, either your own, or a lightweight open source one.

3) Seperate application logic from markup

Now doing this may sound simple, but doing it *well* will inherently move you into a situation where you'll be using an MVC or a templating "engine" like Smarty.
This will look much prettier and give you (or a non-techy designer) complete freedom to change your design without touching any code that could potentially break the application.