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If you find one you enjoy playing, set yourself up a budget for spending and stick too it. A good limit is what you would be spending on subscription games. If you need to spend beyond that, I would suggest you look for a different game.
Stay away from the big ticket items, they generally are very bad investments in what you get out of a game no matter how cool they look.
If you have a hard time keeping to this rule, remember that the f2p market is very crowded and competitive, the chances of any of these games folding are high. Won't you look the fool if you have wasted a lot of money on one that does.
I welcome any other suggestions to this thread.