If you're using [b:caad4f384f]aluminium[/b:caad4f384f] as it seems, you should use a wire that is [b:caad4f384f]1/3 of the trunk's (or branch's) diameter[/b:caad4f384f].
If you plan to use copper, you might want to use some strong equipment as thick copper wire is almost impossible to be bent by hand.
So I'd suggest to go with thicker aluminium and bend the trunk in small amounts at a time. First just form a basic look and then gradually increase the sharpness of the curves.
PS. I'm sorry if this wasn't too easy to understand, I'm from Finland.
