SEO is a tricky business that can be very easy to get wrong.
Search engines are constantly changing the parameters in terms of how you achieve the best possible result to be found online.
This is why it is important to optimise your content. Google and Bing need to crawl your website when a user enters a search engine. This helps find the most relevant matches, and put your website front and centre in front of that user.
There are some elements that have been clear for a long time, such as making sure that your content is up to scratch.
What to do
Keyword stuffing, or simply listing your keywords, is a bad idea. Search engines are intelligent enough these days to realise when websites are simply trying to deceive them by listing their keywords. They need to be interspersed throughout your website in properly written English, rather than stuffed in there.
User experience is the most important aspect to your website.
Design elements such as background colours being the same as the text are frustrating for people visiting your site. The content accidentally being placed off screen through poor design could also adversely affect you. They will click away from your business if they experience this.
Your links should also contain anchor text that is relevant to the site or page you are linking to.
Bolding or italicising every keyword is also looked down on. If there is a context for it, then you should not have a problem, but to use this tactic for every one of your key words is likely to have a negative SEO impact on your site.
Google penalises certain websites if they send out a lot of spam. This is why you should never link to these sites, as this can again affect your SEO.
These issues may seem subtle, but there is no point negatively affecting your SEO if you can avoid it!