When considering a tattoo shop, there are a few things you should keep in mind. Take into consideration that tattooing is like a minor surgery: look for cleanliness in the shop and work-stations, including the floors, the counter surfaces, and the chairs/tables you will be seated on. There should also be no smoking in the studio; aside from this being blatantly wrong, this promotes cross-contamination and can transmit disease.
If the artist you're considering happens to be working then watch him/her. Look for barrier film covering all contact surfaces, and a clip cord (the cord that comes out of the tattoo machine) protected with plastic. Some studios have autoclaves for re-usable equipment, such as tubes and grips, while others may use single-use disposable tubes and grips. Either way, the tattooist should open pre-sterilized packages of single-use needles and disposable grips, or (in the case where equipment is sterilized by autoclave) pre-sterilized tubes and grips, in front of the client before the tattooing procedure is performed.(Do you see the artist open a sealed bag each time he/she changes a needle in the machine?) Are they wearing gloves throughout the procedure, and do they change them whenever they touch an unprotected surface (i.e. the phone) before they go back to their client's procedure? It is not uncommon for an artist to go through several pairs of gloves during a session.
Obviously, you want to look carefully at the artist's portfolio. Do you like what you see? Are the photos taken well? If you go to a shop and they have no portfolios to show you, do yourself a favor and leave. No amount of verbal swaggering by a guy at a counter will give you any idea of what you're getting yourself into!
Cost is an equally important point: there is a fair price for everything! Rates vary from location to location, but, as the old adage goes, "good tattoos aren't cheap, and cheap tattoos aren't good." Just because someone might give you a "good deal" it may become more expensive in the long run. Tipping is optional, and should be determined by your satisfaction. Don't get robbed!
In the end, the best advice is to use simple common sense: ask questions and get information, and ask around. Often, there are many that you might know that have had some work done at the same tattoo studio you are making inquiries about. Their insight on to how good (or bad) the experience was for them can give you some insight on your tattoo experience will go for you. And possibly the best way to judge where to get your tattoo done is how YOU feel about the tattoo studio you are considering: if you do not feel comfortable with the studio, the staff and/or tattooists then keep looking!