Edit wp-config.php and add
define('ENABLE_CACHE', true);

Create the directory cache in the folder wp-content/ (take care it’s writeable!)

If you want to take control about expiration time, you can do this including these lines in wp-config.php:
define('CACHE_EXPIRATION_TIME', 900);

That’s all : )
The next time, your blog is being visted, the pages will be cached..
B.t.w.: You can empty the cache directory every time you want (in case of trouble, etc..).

More infos:
http://www.mitternachtshacking.de/blog/356-wordpress-cache-aktiviert (de)
http://sw-guide.de/2007-07/bessere-blog-performance-dank-wordpress-internen-cache/ (de)

Update (!):
The file-based caching mechanism has been removed from the WordPress 2.5 code. Detailed infos at neosmart.net.