Dress well and be prepared for any questions the landlord may ask. If you are not confident in English, try to bring along someone who can help out. As a foreigner, you sometimes need to be extra convincing.
If you take a day to decide, you might well find your dream flat already taken by someone else. Don’t let the competition scare you into signing a contract unless you are sure, but when a good flat comes up, grab it!
If possible, try to visit the area around the apartment both during the day and at night, or ask around to see what it is like. Perfectly peaceful areas during the day can turn into unbearable residential areas at night if there are bars nearby. Likewise, a calm neighbourhood in the evening may be a nightmare during the day due to traffic or construction works.