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---> Avoid having pages that don't have much value (unique, original content).
---> Avoid having pages that are too similar to other pages on your site, or elsewhere on the Web (except for reprints, direct lifts of historical documents, that sort of thing).
---> Stay within the published guidelines of the SE's. (Like it or not, the SE's are not inclined to spend time crawling sites that run afoul of their guidelines, so you might as well know what they are.)
Here are some ideas to get you going on positive things you can do to get your site in great shape [webmasterworld.com]. It's Brett's 26 Steps thread - quite possibly the best short guide there is for getting a great Web site off the ground.
Here are some ideas to get you going on potential issues sites can suffer from [webmasterworld.com]. It's a thread that focussed on dropped sites but it's a decent checklist more generally speaking too, if you feel you're not getting enough attention from the SE's.
Hope that helps. ;-)