The best way is to use 301 redirects in your .htaccess file, the 301 code signals to google that the url has been permanently redirected.
I'd also recommend signing up for Google Webmaster Tools and submitting a sitemap to them, if you haven't already, as this will help them to understand the changes you're making to your site.
redirect 301 /old-url http://www.yoursite.com/path-to/new/page.html
This article from teknocrat and this query should give you some more information