The problem of those isn't so much to make them fair and balanced - that's easy enough, especially if you allow ties.

The problem arises if you're trying to think of items that represent your choices, in a way that every outcome seems obvious.

I did it recently for my game with 7 elements (allowing two ties for each element, two "strengths" and two "weaknesses"), and it's really difficult.

I mean, check out the systems in the link Jibb posted. Does "Human" beat "Air"? (it does, apparently). Does "Sponge" beat "Gun"? Who wins, "Wolf" or "Devil"?
I'm pretty sure the website was more concerned about possible, distinctive hand-gestures than logical outcomes.

In the end, sometimes it might make sense to add an element with no "real-life-analogon", or something that could easily be strong to anything. In my case, I went with "machine", which, well, isn't really very well-defined in the context of the game, so I can get away with it being weak/strong against unusual items.
That made the system fair, so I think it's okay.