Shared taxis are cheap and plentiful. Stop taxis and tell them where you want to go or tell the passing taxis by pointing, and they will either accept "na bai" I'm going, or decline "nÃ£o" no if people, already in the taxi, are going to another part of the city. Prices are more or less fixed, and you'll normally not get cheated, except when you're an obvious foreigner and taking a taxi from the airport or Hotel Bissau Palace.
Group travel is cheaper, for example:
1 person 300 FCFA, 2 persons 300 FCFA, 3 or 4 persons 600 FCFA
There are also minibuses "toca-tocas" going on fixed routes, you'll pay 100 FCFA a person for the ride, but the buses are often very crowded!