There are many facets to SEO and search engines continually change their methods of indexing sites to help prevent same sites from staying up in ranks all time.
Here are some strategies we have personally used with our web design firm:
1) Google AdWords. This is an excellent and cost effective method of getting good search results. You set maximum daily amount you would like to spend on your key words and phrases. Within first month we were getting more client contacts through using AdWords.
2) Overture Keywords. A lot like AdWords, but we found that their system did not really allow you to set a daily limit to how much you wanted to spend and it quickly got out of hand with some of phrases we had chosen! Still, pretty cost effective in grand scheme of things.
3) Yahoo Regional Directories. Once you are able to get your site indexed by Yahoo Regional Directories, I think you stay in there for good! We've been in there for years now and continue to get clients who found us through this directory. (And it is free to submit your site)
4) Keywords / Description / Alt Tags / Title Tags / Page Content. All big SEO companies rely heavily on these 5 things - all of which are contained in code of your web pages. However, with a little research, you could do these yourself (or have it done by your web designer). A few great tools to help you with this task are: Overture Keyword Selector Tool (http://inventory.overture.com/d/searchinventory/suggestion/), Google Suggestion Tool (http://www.google.com/webhp?complete=1&hl=en), Meta Tag Analyzer (http://www.iboost.com/tools/meta_tag_analyzer.htm), and Keyword Density Analyzer (http://www.keyworddensity.com/).
The main benefit to hiring a professional SEO company is that they work for you to constantly update your keywords, etc. and submit your URL to search engines. They know exactly how many of each keyword to use, etc. And this is often a service worth paying for if optimization is a real concern in your industry. A good SEO company will also stay up on what search engines are doing and change their tactics accordingly. They will also never use an "illegal" tactics that could get you banned from search engine. Be wary of companies that "guarantee top 10 placement". I recommend http://www.submitsolutions.com.
5) Online Directories. Submitting your website URL to free online directories is a great avenue of promotion. One way you can find directories is to use Google or another search engine and type in "(type of your business) online directories". Some that come up may ask for a small amount to list your site. If they do, you should always contact them and ask them for more information such as how many unique visits their directories get per month to see if price is a good value.