There are several ways you can get more credits. The first is to answer requests. Each answered requests gets you 1 credit.
The second way is to invite friends to join StaffTraveler. For every user that joins StaffTraveler, you receive 2 credits.
Another way to earn free credits is to share tips on your favorite places with your fellow StaffTravelers. Go to the Tips tab to share your tips.
Lastly, you can purchase credits. Go to the More tab and choose 'Buy credits'. You can pay with your Apple or Google account.