I like to use Google because Craigslist eventually blocks or gets those search sites shutdown but it can't get Google shutdown. For example, if you are looking for '57 Buicks you can enter the following strings in the Google search box to search all of Craigslist:
1957 buick site:craigslist.org
or
"1957 buick" site:craigslist.org
The first string finds anything with both 1957 and Buick in it and the second string finds phrase "1957 Buick".
You can also use these:
57 buick site:craigslist.org
"57 buick" site:craigslist.org
I use all four of the above strings. Also see this thread:
New way to search Craigslist