Just read the google guidelines especially the 'Quality guidelines - specific guidelines' on http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=35769#3
There's one thing that may be really harmful for the whole SEO process, that's any kind of spamming so if you want to see positive results, just think more of quality of your work good luck to you!
What has been suggested already, plus I am sure getting a lot of low quality links to a new site is committing seo suicide as well.
There are lot of things considered as negative.. like building backlinks in bulk, duplicate content, bad neighborhood, keyword spamming etc.
If you are getting back-links from sites like porn and gambling related sites to your entertainment site.
You can keep in mind while doing SEO is that do not do any spamming and not to indulge in black hat SEO.
Putting Keywords on page in same as background color i mean , and as early-ear said keyword stuffing is also considered as Black-hats