I strongly recommend Cristina and André Yellow House in Principe Real. Although the street can become noisy at night (but a pair of ear plugs do the trick) the place is comfy, with everything you need for a short or longer stay. It's ideally situated in the heart of Lisbon, 20 meters from a bus line that brings you in 3 stops to a metro station. A supermarket, lots of bars and restaurants are close by… even two lovely public gardens to enjoy the late afternoons. More over, Cristina is helpful beyond expectation - sending me notices by mail when something interesting was going on nearby.