There are a lot of ways to promote your website and, unfortunately, a lot of these methods are mistakes. Here is a list of some of the more common mistakes (often referred to as Black Hat SEO) that you should steer well clear of.
1. Bad Neighbourhoods
These sites are also known as free for all (FFA) pages and link farms. They serve no other purpose other than to list tens of thousands of unrelated websites. Not only will these sites not provide your site with any traffic, certain search engines will ban sites who participate.
2. Over Optimization
Optimizing your web pages for a particular search engine can be a good thing. Over-optimizing can defeat the purpose, however. Search engines are catching on quickly to pages that appear 'over-optimized'. Stuffing keyphrases into your pages is the most common problem. Never use hidden (invisible) text to add keywords. You will get caught, and your site will get banned.
3. Doorway Pages
Doorway pages are orphaned web pages that are typically optimized for a particular keyphrase and a particular search engine. They are promoted to the search engine in an effort to achieve high rankings. They are not linked back to by any other page on the website and, as such, search engines are able to flush them out quite easily and penalize the entire site.
4. Traffic Generation Scams
Services that purport to drive lots and lots of traffic to your website are typically not very useful. The traffic tends to come from seedy places like domain names that have lapsed in payment or commonly misspelled domain entries. Most of the traffic that hits these pages is accidental and is therefore of no value to you. Other traffic may come from other participants in the program who are required to visit other websites to gain 'credits' for their own site. Again, this traffic is not targeted and therefore of no value. Some traffic may even be generated by automated software!
5. Shadow Domains
Shadow domains are small, optimized, supplementary websites designed to drive traffic to a different website. The way this works is that an SEO company will design and optimize a website on your behalf. The danger here is that the you do not own this new website; the SEO company does! This means that if the relationship sours, they may choose to redirect this site's traffic to whomever they choose (see #4 above), or even sell the site to one of your competitors! Be very, very careful of this technique, as some supposed search engine 'experts' even use this method (no naming of names here, however).
One final note: Any company that tells you that it can achieve 'guaranteed' rankings is either lying to you or using some of these Black Hat techniques. Unfortunately, there is no way to guarantee a number one result within (unpaid) search results. When choosing a search engine optimization company to work with, be sure you feel comfortable with them and trust them (e.g. money back guarantee). And if you decide to promote your website yourself, consider yourself a little better educated against these common mistakes.