I'm sure someone will chime in with a permission slip template, as several types/wordings have been posted on the forum over the years. But I want to precede that by advising you to be careful of permission slips in general, unless the property owner themselves has said they want something in writing. Reason is: If you show up at a perfect stranger's doorstep with a paper for them to sign, you're going to get poor response. Better to just talk casual, under the auspices of "historical research" (or whatever) interest, and was wondering if you could walk around, for purposes of an "article you are writing" etc... (afterall, you'll post the results on T'net, right? haha).
Like I said, this pyschology is only for if this is just casual. If you've got a specific cache, or specifically a person asking for something in writing, only then is written agreements, signatures, insurance, blah blah blah, merited.