Protocol for slipstreaming another cyclist?
I've found it courteous to come up beside the person and ask permission. I usually say "how far you going?" or "mind if I join you?". The real key is to take turns drafting. If the person says yes, I  usually take the first pull in front just to show them I'm not a wheel sucker. 

As an avid cyclist, I've found very few things more annoying than riding for a good chunk of time, only to look back and find someone just sitting on your wheel with no announcement or indication that they are there.