Don't just stick to surveys as it can be very hit and miss if there are any available for you, and will be very dependant on your demographic.
Use the 'free trial' offers to (e.g. Credit Expert for UK users), as long as you are confident you will remember to cancel before you start getting charged.
Also look out for any paid offers for things that you would be spending your money on anyway, e.g. inkclub.com I've used 3 times for printer supplies (1250 credits each time) and I just needed new glasses so got 2500 credits for simply clicking through to glassesdirect from offer page rather than going to it via Google. Clearly, check you are happy with what's on offer from these providers before choosing to use them over anyone else just for the credits. But, if you are, you can get far more credits this way than from surveys.