Ampario surveys are good to get credits. Samplicio very rarely has a survey, but still try it just in case. As has been said, you've got to be patient and persistent with the surveys as you won't get them every day (and sometimes for maybe a whole week or more), and when you do you may not qualify. You may sometimes spend 5-10 mins doing one only to find that you've failed. But they will eventually pay.
I also signed up to CreditExpert for which I got 1750 credits. Just make sure you cancel before the 30 day trial runs out otherwise they will start charging you as you havce to give your credit card details when registering... that is of course if you don't want to continue it.
Then there are a few minor surveys and free offers which are once only and pay 75 credits. But still, every 75 credits adds up and gets you to where you want quicker.
There are also offers which may cost, but if it's something that you need (e.g. printer cartridges, flowers, photo printing, etc), then that may also be worth considering.