The first one is to always do your dailies. Many of them are ridiculously easy, and some offer up to 70 gold. You can have three outstanding dailies at any one time.
The only other way is to win games. Like some other games, three wins equals a gold reward, which varies according to some formula I'm not aware of (or maybe it's just random). Three wins also brings at least one new card (and, rarely, an entire pack).
If you find yourself with some extra cash, I really would recommend the campaigns. Not only are the stories pretty good, but moving through the chapters leads to new cards (which are often powerhouses). I can't remember if the campaigns also reward gold ... I'm trying to picture it, but I played them all right away, so even the latest one was a while ago.
Personally, I've found the rewards in ES:L to be fair. Of course, I paid for the campaigns, which helped keep my card collection pretty robust.