Map your DB, envision all possible interactions
if you do no use all of them today; later you might be glad that you did think about it
such an error could cost you at least two days of hard labor .. I know I did it :)

One of the most important things I think of in a website is logical structure, especially with PHP and other server-side technologies. I can't tell you how many sites I've used that had dependencies upon dependencies upon dependencies, literally dragging you around in circles to find where some variable is REALLY being set or some object is REALLY being defined.

The most important tool for designing a web site is a pencil and paper.

Get your structure laid out before you even touch a keyboard.

Keep SEO in mind when creating page titles and descriptions but for page content concentrate on what is good for the visitor.

In addition to your comment "Keep SEO in mind when creating page titles and descriptions", I'm trying to find out whether I should structure a portal site I'm doing SEO for as one domain name with a tree structure (mydomain/theatre, my domain/gigs), or have a separate domain name for each category (www.theatreguide, www.gigguide).

Any thoughts?

Whilst on the subject what is your view on whether keyphrases help seo if part of domain names - I am getting conflicting advice.

I would say to structure everything correctly I take time to plan this first, for example I make use of the PHP include feature alot and put in lots of folders eg (http://php.example.com/menu/nav_1.php) so I can quickly and easily change or move things and put in some sub domains to keep everything tidy for example I would use css.example.com/ for css and the same for javascripts (js.example.com) etc.

Then the obvious database work to structure that correctly, this allows you then to concentrate on the html and the layout rather than at that time the content, ensuring then that you take into account size of browser / SEO / alt tags / cross browser compatibility etc..

In respect to SEO I personally would use one domain, this would be better because deep links into the site on the sub domain benifit Page rank on the whole site.