Telling your user-agent (web browser) to use Polipo is an operation that depends on the browser. Many user-agents will transparently use Polipo if the environment variable ‘http_proxy’ points at it; e.g.
$ export http_proxy=http://localhost:8123/
Netscape Navigator, Mozilla, Mozilla Firefox, KDE's Konqueror and probably other browsers require that you configure them manually through their Preferences or Configure menu.
If your user-agent sports such options, tell it to use persistent connections when speaking to proxies, to speak HTTP/1.1 and to use HTTP/1.1 pipelining.