Well... if your selling the sig. Have a watermark accross the sig saying preview or something. If its just for you. Have text in low opacity somewhere, somewhere that is not that obvious. Then the guy who is looking for rips goes "hey, a sig without text, an easy target" and bam you got him, he gets banned from loads of sites.
Make bad sig! No one rips those... Blend your name into the background. SAVE YOUR PSDs and like MADMAV said put some watermarks or very faded text into the background.
my only advice to you is that they'll be cheating themselves. IMO Those rip signs are a hassel and takes out the fun in sig making less your giving out .psd's to the community.
Write your name on it in a place that, if they tried to erase it it would make a drastic difference. Meaning, dont write your name in a 4 size font in a black corner; its easy to steal.