How do I work with prims that AREN'T SQUARE

Like this: http://www.ngsam.com/helm.jpg How do I move that yellow piece (It's a sphere) over to the other side so that is is symmetrical?

If you mean make the same piece to the other side to make it simetrical the both sides you only need drag and copy the same piece to the other side and turn on 180degrees in the oposite side or, you can copy ot the same prim and put the same values from the other prim but in negative values.

180 degrees on which axis? here's what happens when I shift drag the yellow piece over: http://www.ngsam.com/helm2.jpg

I tried rotating 180 degrees on one axis, then tried doing it on all of them, and it still wasn't symmetrical on the main piece. I can rotate the main piece, but then that doesn't really seem to get it perfect just free rotating it.

It looks to me as though the main piece isn't lined up square with the gird, so when you drag the yellow piece across it looks asymmetrical. For things like that you probably could work out exactly what co-ords the pieces need to be at, but I find it quicker and easier to do it by eye.

Get it close and then adjust using the number boxes on the object tab, changing the numbers by smaller and smaller increments until its how you want it.

Take a look at this.

http://www.wishland.info/varios/helmt1.jpg
http://www.wishland.info/varios/helmt2.jpg
http://www.wishland.info/varios/helmt3.jpg
http://www.wishland.info/varios/helmt4.jpg

I just make like I say in my previus message and Wulfric says.