Ask if you can bring your firearm, most likely you'll be told no, but I haven't ever had a problem with a civilian bringing theirs riding with me. Don't dress stupid, don't show up wearing anything that will embarrass the deputy, i.e. big Johnson t shirts...
Do what you are told the whole time, if the deputy tells you to stay in the car, STAY IN THE FREAKING CAR. The deputy will probably not mind questions, and this will normally make a shift go by faster. Also keep in mind you are riding with a K-9 unit, you need to ASK BEFORE trying to pet the dog. This seems like common sense, but you'd be surprised at stories I have heard. (I'm not a K-9 officer) Keep one thing in mind, some officers/deputies don't really like ride along's, so do your best to be friendly yet professional. Contrary to popular belief, Officers/Deputies are not robots, we have good and bad days like everybody else, just bear with whomever you are riding with. He/She may not want to talk about just LEO related stuff either, mix up the conversation with other topics too. Also be prepared for the amount of paperwork any LEO has to do, a lot of people forget or don't consider this when going on a ride along, for every arrest, traffic stop, K-9 use, or anything, there is always paperwork to be done after the incident is over.