Masking tape leaves a residue behind if you keep it in place for more than a day. And some cheaper tapes just leave glue behind anyway.

Blue-tack or similar stuff might be one of your best options, because you can always shift that. Just make sure that it's stuck to a varnished area, because it leaves grease spots (and occasionally bits of itself) behind if you stick it to unfinished wood. If there's a plastic pickguard you can stick it to, so much the better.

Graham.