When upgrading from SharePoint 2007 to 2010, you’ll need to upgrade your master pages (if they’re customized) to take advantage of the new features in 2010, such as the ribbon. The first question is to decide if you want to keep the current master page and add all the new controls in, or to start with a new 2010 master page and customize that. We decided to start with a new master page based on the 2010 master page, v4.master, and customize that. Here’s some resources I found useful when creating the new custom master page for our site.
If you choose to keep the 2007 master page and add the new controls into it, this post lists all of the steps:
http://msdn.microsoft.com/en-us/library/ee539981(office.14).aspx
If you plan to start with a new 2010 master page, a starter minimal master page can be found here:
http://startermasterpages.codeplex.com/
I also found the following pages invaluable. This blog post lists all of the base css classes in the SharePoint master page.
http://erikswenson.blogspot.com/2010/01/sharepoint-2010-base-css-classes.html
This blog lists the styles you’ll need to modify to change the look and feel of the menus:
http://blog.drisgill.com/2009/11/sp2010-branding-tip-7-using-simple.html