Like a house, a website needs regular upkeep. Don't assume that once the site is online, it's finished. If people are to return to the site, it will need to have fresh content in order to make the site "sticky".

There's also e-mail to answer, links to check, and re-submissions to search engines once information has been updated. For a small site, this can take as little as two or three hours a month, medium sites are more likely to be three hours a fortnight, whilst large sites can have someone doing full-time maintenance on them. So make sure the cost of maintenance is included on your budget.
If your site does not need to be updated too regularly, and you're feeling quite confident with your newly developed skills, it maybe a good idea to invest in a good Search Engine Optimisation and Submission software such as WebPosition Gold.

If you're working with a web developer who is helping with aspects of your project, the company may offer a maintenance agreement as part of the contract. Ask about such an agreement at the beginning of the project. Working with the same people throughout will ensure consistency; a new person or company won't have the same working knowledge of your site.