The best piece of advice I ever read about creating a website / optimising a website was years ago:
make sure all your pages link to at least one other in your site
This advice is still sound today and the most important piece of advice out there in my opinion. Yes it’s so simple it’s stupid.
Check your pages for broken links. Seriously, broken links are a waste of link power and could hurt your site, drastically in some cases. Google is a link based search engine – if your links are broken and your site is chock full of 404s you might not be at the races.
Here’s the second best piece of advice in my opinion seeing as we are just about talking about website architecture;
link to your important pages often internally, with varying anchor text in the navigation and in page text content
…. especially if you do not have a lot of Pagerank to begin with!