When I first joined, I played DT's they make 50 clintz for what place you make, I believe.And the better you do the more you make when your in the top 100 or so someone please correct I haven't DTs in forever
Daily Tourneys are a good way for making Clintz and the higher you finish the more money you make.
Otherwise you can try and play the market depending on your starting base.
It requires patience but you can pick up bargain priced cards then immediately re-sell them on to make a small profit, but you have to be really quick and also know your card prices well.
Finally, buy some credits and get loads of packs and sell what you dont need/want - simples