Here's some Do's and Don'ts for when you visit a maid cafe, or if you see a maid on the streets.
1) Don't touch the maids.
You'd think that this one would be obvious, but many tourists seem to think that the maids are some kind of playthings that you can just throw your arms around, poke, or try to feel the materials of their dresses. You don't want a complete stranger coming up and poking you, and neither do the maids.
2) No photos.
This is an easy one to misunderstand. You'll see a number of maids out on the streets around Akihabara, handing out fliers advertising their cafes. As a tourist, it's natural to want to have your photo with "one of the cute little natives in the funny dress". Doesn't work that way. The cafes make some of their money by selling photos within the shop, and if you take a photo on the street for free, it's money that the cafe doesn't see. If you really want your picture taken with a maid, go into the cafe and pay the 500 yen for it. Not only will the maids pose happily for you, but they may even spend 5 minutes signing the photo and adding little drawings to it.
3) Don't stare.
Yes, maids are dressed up in fancy outfits which are designed to attract attention, but just staring at someone is creepy. Don't do it.
4) Don't tip.
Tipping is not a common practice in Japan, and the wait staff is paid well to begin with. So, it's ok to not tip at the end of the meal.
5) Do have fun.
Maid cafes are intended to be entertaining. Relax and just follow common sense.
6) Do watch the other customers.
If you're not sure how to behave, watch the other customers for cues. In general, you'll be guided to a table, showered with greetings of "welcome home, master". At the table, you'll get a menu and a list of the day's specials. To get their attention for placing an order, just speak out "sumimasen!". If you order food like a rice omelet, curry rice or pilaf, you may be asked what you want written on it. The maids "add love" to the meal by drawing hearts or something similar on the dish in catsup, and some places let you choose what you want drawn. Pay at the table. Again, get the server's attention with "sumimasen", and they'll bring the bill to you. Put your money in the tray they bring you, and wait until they bring you your change. Then, when you leave, you may be escorted to the door with a shower of "please be careful, master", or "please come back, master". If a given cafe has different rules, you'll be able to pick up on them by watching the other customers.
Places like @Home advertise English-speaking staff, and they will post online (or at the front of the store) the hours when the English-speaking staff are on duty. Otherwise, many maid cafes will at least have menus in English. If you look like you can't speak Japanese, they'll give you the English menu automatically.
7) Do ask for the points card.
If you listen closely at the end of the meal, you may be asked if you want to get the points card, but not all places offer them. The cards should be free, and they make a nice little souvenir. And, it just might be fun to come back to the same place once or twice in order to get the "service" from the card. No, "service" doesn't mean what you think it does. It's just a free present for spending 5000 yen or so at the same place. You can expect that a present will be something like a free photo with one of the maids, or a free drink.
8) Do get your photo while you're at the cafe.
As mentioned above, the cafes make some of their money by selling photos. For 500 yen, you can get a unique souvenir with yourself posing with one of the maids. These maid photos are called "cheki", which is the Japanese version of the "click" sound the camera makes when the photo is taken. Generally, "checki" are little 2"x3" Polaroid photos embellished with hearts and writing by one of the maids.
9) Don't go drunk.
Alcohol is great for making people do stupid things. If you're not careful, you can get yourself kicked out of the cafe for harassment, or buy yourself some time in jail. Maid cafes are not suitable places for going in drunk. Stay sober, have fun. (On the other hand, there are maid bars and many cafes do serve some alcohol, so don't drink yourself stupid in these places.)