It is difficult to make a top5, but here are several things you should take account:

1) website internal linking structure:
- do not use javascript or flash links
- use appropriate text-anchors for your links
- you may use "nofollow" tag to your non-relevant pages if you want to concentrate PR to the interesting parts of your website

2) use different page titles and descriptions for your site pages ( do not use the same title and description for all pages )

3) use search engine friendly urls ( yoursite.com?state=florida&property_type=houses will become yoursite.com/florida/houses ) - this could be done using .htaccess

4) use h1, h2 and h3, also use strong, bold and italic tags appropriately

5) use a good density for your keywords / phrases into pages

6) be careful with your outgoing links ( links from your website to other websites )

7) obtain valuable ( in terms of PR and / or trust ) inbound links ( links from other website to your website )

8) do not practice seo spam!