by public transport
When renting cars in Berlin or Germany for that matter, two of the best sites for car rental deals are:
Unfortunately, both are only available in German, but I have found, especially with billiger-mietwagen.de, the best deals available. For example, recently during the summer holidays, using billiger-mietwagen.de, we rented a midsize Opel 4-door, for two drivers, including unlimited kilometers and full insurance coverage, over 14 days, and payed 400€. The actual rental company was Avis, but Avis did not offer the same deal directly.
Otherwise you can contact the international rental agencies directly.