I've been working over Labor Day weekend a bit, reviewing sites for clients (and friends) and continually find that site architecture best practices are being ignored. Since Monday isn't a holiday globally, I thought it would be worthwhile to provide a quick peek into what I'd recommend on the structural side of SEO:

• Dynamic URLs - If it's even remotely possible, avoid them completely. From what we've seen, a couple of the search engines actually have different trust metrics or ranking criteria they apply to dynamic URLs. It's not as big a deal for Google as for others, but why take a chance? ISAPI and mod_rewrite are simple to implement and certainly worth the time.
• 3 Clicks to Any Page - Normally, webdev industry insiders consider this a rule for usability, but it's also critical to successful SEO. If you want spiders to quickly find your content and engines to rank it well, make your sitemap page accessible from every page on the site, and if you've got a monstrously huge site, use sub-sitemaps for unique sections to ensure that thousands of pages can be accessed in 2-3 clicks/links.
• Avoid Unneccessary Subdomains - It's up for speculation as to whether each of the engines applies the entirety of a domain's trust and linkjuice weight to subdomains. Some think it's on a case-by-case basis, which I find reasonable, and others thing they are generally devalued as compared to the primary domain. In either case, unless you're looking to dominate the SERPs via a subdomain takeover (like this guy), subdomain content can easily go in a subfolder.
• Internal Anchor Text Bombing - The funny part about this tactic is - it used to work. You could change the link to your site's home page to read "denver mortgage refinance" and actually rank for it. Luckily, Google & Yahoo! got smart right around the same time and actually started penalizing sites that used this tactic. Your best bet now is to write internal anchor text for visitors, not engines. If you run a Denver real estate site and link to your refinance page, it might be fine to use that anchor text, but for primary site navigation, this technique is more likely to hurt than help.
• PageRank Flow - Two words will suffice - ignore it. PageRank flow through a site used to be a valid tactic, but these days, you're wasting valuable time determining the number of links, where they point and attempting to modify your site based on the 7-year old formula. Similarly, keeping a careful eye on your outbound links doesn't pay - just think of the human user and deliver what they'd want (it's remarkable how far this will take you).
• One Piece of Content, One URL - This probably trips up more big, commercial sites than any other. The issue is that the same content is accessible in multiple ways and on multiple URLs, forcing the search engines (and visitors) to choose which is the canonical version, which to link to and which to disregard. No one wins when sites fight themselves - make peace and if you have to deliver the content in different ways, rely on cookies or session IDs so you don't confuse the spiders.

Hopefully, these suggestions are already issues you're familiar with and apply without a second thought. If you've got others to suggest, I'm all ears (even though I'm technically taking the day off).