I have it on my services page - I figure most guys are going to check there at some point before booking. On AW I have it near the bottom of my profile in bold caps.
I say "Some or all services may be refused if you have poor hygiene or sexual health. I will not see any client under the influence of drugs or alcohol." Might not be word for word there, but that's the general idea. There are some who just don't read it though, I've had guys trying to book while drunk/high before, and one guy who asked me if I could arrange for him to buy some coke at the same time! The best thing you can do is keep an eye on the way they act while booking, particularly if over the phone, and then be observant when arriving. I had a horrible experience with a man who didn't seem drunk at all when I arrived - but he was holding a pint and there were empty bottles in his room. Now if I turned up and a client had alcohol visible, even if he seemed normal and sober, I would walk.