Its really depends on your experience and how important the site it. If the site is very important you really want and expert to handle it.

The major point is what temi said about the type of site, yes it's true if the site is expensive and important then you should not experiment on it.

You can experiment on other sites which are not too important to get skilled with seo before handling any big site. It can harm your site as well.

Otherwise you can also learn yourself with internet resources, books available.

SEO or Search Engine optimization is a procedure or a principle in helping a certain website increases its traffic, improving its page rank and giving your website more chances of getting the attention of the researchers.

The need of SEO is to increase the amount of visitors to a web site by ranking high in the search results of a search engine. The higher a web site ranks in the results of a search, the greater the chance that site will be visited by a user.

SEO is now a days a booming IT enabled industry.....

It depends how big the site is and on the work loads. Our company hires SEO Specialist, all of them are made outsourcing. Saves money and worth the price!

its depend on time if u have time i suggestion you "have go you yourself"

If you have Time and some knowledge do it yourself
If you don't have time but have money hire an expert
If you don't have both just forget about promotion.

There are plenty of resources available for do yourself SEO.
There are so many SEO Experts who could be hired
But there are really very few or none who can work without reward (i.e fee)

Regards
John