There's lots of helpful good guides on this in the guide section of the classic forum. However The one tip I would give that you don't hear much is this: If you have pc/internet access log on to classic via fb and look in the gralat pack section and click into gralats via fb offers. Then there's usually a half dozen or so downloadable apps for your idevice or other smart phones. Those can get you 500-3k gralats I would avoid the surveys as they can be long,tedious,and result in you getting unwanted spam in your email.
But the Facebook offers are a great and over looked way to pick up some extra gralats here and their. I log into my account via FB about once a month or so just to check it. Just an fyi I was dead broke on Cinco de Mayo and was able to get an extra like 15k from the fb offers that I used to buy the limited availability holiday items.