Hi!
there are several options depending on the situation.
- The new site is essentially redesigned but maintains the URLs structure > the links will still work (I know it is now your situation, but 1 to consider);
- New site, same domain name, same content, but different URLs structure > 301 the old URLs to the matching content new ones via .htaccess (assuming the site is hosted in an Apache environment);
- New site, same domain, different content structure and different URLs structure too > 301 the old URLs to the most related content page of the new site
- New site, new domain name, same content but different URLs structure > as point 2, but in this case you can also use the "Change of Address" option in Google Webmaster Tools (under "Site Configuration"). I think that Bing webmaster central offers a similar option
- New site, new domain name, different content structure and different URLs structure too > as point 3 but with the GWT option too (and maybe BWC as well).
Hope this helps 