1. Plan ahead for travel because the street will pretty much be blocked off all day.
2. No cars can be parked on H Street after 4am. They will tow you. Around 4:30am they will block off intersections. After that you'll see cars on the street, but (unless they are art cars) they will be for vendor set up prior to the Festival. The street will not reopen until 11pm, so plan accordingly.
3. I tend to pretty much spend the whole day at the Festival (12-7pm), so I am always careful to remember sunblock. Saturday's forecast calls for a high of 89°, so plan accordingly.
4. Bring a water bottle. In recent years DC Water has had a free water station for refilling your bottle.
5. Bring cash. Not everybody takes cards & the ATMs might run out.
6. The H Street Festival is DC's largest neighborhood based festival. In recent years it has drawn in excess of 100,000 visitors each time. I recall that a few years ago lots of people had trouble making cell phone calls. I'm not sure if we overloaded the system, but there seemed to be a lot of missed connections. It can also be a little loud sometimes, and this can make it hard to hear your phone wringing. I suggest making advance plans about where to meet friends.
7. The larger crowds meant things got pretty congested in some spots a few years ago, but stretching the Festival all the way down from 4th-14th and H Street (it used to run from 8th-14th) really helped. I did not find the crowds overwhelming the last two years.
8. Leashed pets are welcome (there's even a special dog area), but make sure your pet is comfortable around crowds. Remember to water your pet on a hot day. Last year we brought our dog Jack, but even with water the black one year old puppy was ready to head home after about 45 minutes (and that was before the Festival even got crowded).