I sign my canvases inside the image on either the bottom left or right corner, depending on what is the least distracting.
I use a metallic marker, either silver or gold, again depending on the image. The metallic ink is nice in that from certain angles it really stands out and from other angles it's nearly invisible.
I think the signature should not call attention to itself.
For matted prints, if you sign the mat I think it's best to also sign the print underneath; I think the print itself should be signed in all cases.