Lot of Jiu Jitsu addicts will find a BJJ academy when they are traveling for work or on vacation to "drop in" on a class and most academies will be very welcoming in having a guest student. As a guest student, you should adhere to the rules and regulations of the academy. Finding the right academy to train at is probably the first thing you should research (Friends, Facebook, and Yelp are a good start). Once you find the place you want to drop in at you should call ahead and get some more info on the drop in class.  Here are some do's and don'ts:

1. DO call ahead and introduce your self and let them know that you are in town and looking to drop in on a class.
2. DO ask if they have a mat fee and when the classes are. (Fundamentals, Advance, Comp Class, No Gi, Etc...)
3. DO let them know where you train, what level you are, and  how you heard about them. If you were referred by someone at the academy or know someone there they may not charge you a mat fee.
4. DO your research before you show up to a class. If you call them multiple times and they don't pick up the phone that's probably a very un-professional academy. (Not assuming that all academies who don't answer their phone is un-professional) Read reviews and also make sure they have the class you are looking for.
5. DO NOT call or show up and NOT introduce yourself and be a dick. They don't have to have you on their mats. You are the visitor so show some respect.
6. DO NOT SHOW UP LATE!!! It is most disrespectful to the Professor, Instructor, Students and the academy to show up late, especially as a "drop in" guest.
7. DO ask ahead of time if there is a specific Gi or No Gi requirement so you can prepare ahead of time. (Some academies require you to rent academy Gi's and No Gi gear and have specific color gi's) If that's something you do not want to do then that particular academy may not be the one  you drop in on.
8. DO introduce your self when you step on the mat and don't be a dick.
9. DO expect to train hard because if you are a guest, the students are coming after you. Not with a malicious intent but you are a "new" fish in the sea. Remember, training hard is perfectly okay but DO NOT go ape shit and move like you just downed a double espresso and a 30oz monster energy drink... The mat enforcer is watching and you will pay the price.
10. DO have fun and make new friends, there is nothing like traveling to a new city and meeting new friends on the mat.

The above 10 do's and don'ts are just a  guideline and of course there are exceptions and different circumstances when it comes down to visiting other academies. Once you've dropped in for a class you should definitely leave a online review on the academy and the class, whether positive or negative for future students and drop in guests.