Go to the main search page, which you may want to bookmark in your browser:http://www.ancestry.com/search/default.aspx
scroll waaaaay over to the right and look for a small-typeface link just below the navigation bar. If it says "old search" click on it to ~go to~ Old Search. When you are in OldSearch you can use the link in the same place (which will say "New Search") to switch.
They used to have this link on most search pages but removed it -- probably to try to force people to use the default NewSearch.
If you are on any Ancestry-side page (databases or message boards, but not blogs) you can always find the global search page with the link by clicking on the Search link in the main navigation bar.