Big question!
This isn't by any means an exhaustive tips list.
Very few things are black and white in SEO, and I would hate to call myself an "expert". I think I understand enough to offer "some" advice though. There are others here who I would look to for guidance (Chris

)
You don't say what your site is about. Some areas are easier to get good results in than others. Notoriously difficult areas are things like Real Estate where incredulous SEO companies have spammed the c**p out of the engines.
Do not be tempted to try any tricks such as cloaking or "doorway" pages. You may get short term results, but you'll suffer in the long run. Be ethical. Tricks are exactly that.
Above all build your site for the USERS, not for the Search Engines. Good, useful content, clearly presented. Do that, and you are halfway there.
Before entering in to any PPC or PPI plan you need to make sure that your site is built right.
Copy should be optimised and relevant to target the keywords and phrases that you have identified people will be using to find your site.
The code should be clean. The application of some good coding practice and a consideration for how spiders will crawl your page is a good idea. For example putting alot of javascript at the start of your page may impede some crawlers from indexing due to the resulting size of the page.
Make sure that any links you have on your site are working, and will work without the need for plugins or things such as JavaScript. A crawler cannot follow a javascript link.
If your site uses frames, change it! If you can't (or won't) then make sure that you understand the added complexity involved in getting pages indexed. Ensure that a crawler hitting your main page in isolation (i.e. not in the frameset) can follow links through the site.
Build up some good incoming links to your site from relevant sources such as peer sites or directories. Links in from top directories are worth having. You may need to invest a little money and alot of time in this.
When making directory submissions, look at the directory and determine the best category for your site. Then look at the entries in that category and write a good description along similar lines to those sites. e.g. If they all seem to have 50 word descriptions, don't write a 100 word description.
Most importantly, keep a written record of who you submit to, what you submit and when you submitted it.
Take a look at some of the other posts in this forum. There is alot of good stuff there, as well as links to other fora where lots more SEO people hang out.
Hope that is some help. There is so much more I could add... like I said, it's a big question!
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