I've participated in signature campaigns before but quit when the people running them started requiring that I sign up for their websites to claim the Bitcoin. I'd rather make this as straightforward as possible -- I rent out my sig space and you send Bitcoin to an address I specify. Any takers?
Then don't use the ones that require you to sign up externally. Next to nobody buys signatures individually so just choose one that bests suits you and your posting habbits. Signing up via an external site isn't that much of an inconvenience either and usually takes only a minute.