About the only way, these days, other than surveys, are the mobile videos. 2 credits a pop, you can view 30 of them and get 60 credits in about 12-15 minutes, before they cut you off. You can go back to them and do it again after an hour or so. Seems paltry but they do add up.
True (X) videos are always dried up in January from the "earn free credits" video page here. I am only getting three 3 credit vids each morning there now. Normally get anywheres from 60 to 90 before it peters out, then go back to it 24 hours later. Peak time to max it is right after 3 AM eastern time, by 7 AM you max out with much less. Again, it's dried up pretty much now, but for the last few years it does kick back up after a month or so.
I don't have the patience for surveys myself, but the mobile vids are painless and do add up. I sit right around the 20,000 credit range most of the time and I fund tournaments, send gifts, use Nike balls, etc. Good luck with it!