Went seven times last year. Sounds crazy but the tourist bus is a good starting point. Covers two routes round the city and you can hop off and on as many times as you like, covers all the major sites. Think it's 15 euro for one day and 18 euro for two days. As an alternative the metro and bus services are excellent and for euro can go any distance, single journey or (4.6 euro for a day ticket with unlimited bus or metro journeys). Taxis are also dirt cheap.
