EternalNewbie:
I'll echo much of the advice Clay threw at you and add a bit of my own commentary, FWIW

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1) Make more than one visit to the parlor you intend to have ink you, and watch several of the artists who work there. Most of them have portfolios displaying their work, and that may help you determine which artist will do the best job on the type of tattoo you select.
2) Definitely reproduce the image you intend to ink, and plaster it everywhere. For my part, I photocopied the Sorcere's Apprentice in about six or seven different sizes, and spent an hour taping it to my arms in different locations before I'd settled on a final decision.
3) Go one small increment in size larger than you feel comfortable with. My artist and the parlor owner both told me this, and I regret ignoring their advice. I wish my tattoo were about 10-15% larger than it is, but I have no intention of trying to alter what's existing just to make it a bit bigger now.
4) How clean and sterile is the facility and their equipment? We chose a parlor that is almost pristine, and I wouldn't have considered a joint that looked shabby, dirty, or run-down!!
5) Don't worry about how much working out or gaining/losing weight will affect the tattoo's appearance. Unless you are making MASSIVE body type changes, the tattoo will be fine. My weight has fluctuated by as much as 40 pounds, and my muscle tone has been altered several times in the past four years, and Mickey looks just fine

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6) Take care of the tattoo religiously in the two weeks after it's inked onto your body. Nothing, and I mean
NOTHING, will make a more pronounced difference in how good your tattoo looks than your care for the wound early on. I kept mine vaselined, out of the water, and covered in gauze for about 15 days, and it looked fantastic and vibrant for quite awhile. To that end, put sunscreen on your ink even if you don't normally use such products otherwise. The sun can rapidly age and fade tattoos.
7) I've had mine re-inked once already, and will probably do so again just to keep it looking fresh. This does cause the skin to toughen and swell a bit (permanently) almost like a brand.
8) Finally, know that tattooing is addictive before you decide to get one. People will tell you this, and it's foolish to dismiss their wisdom. I've had to fight off the urge to get more tattoos on several occasions...I'm truly happy with just the one, but some primal instinct kicks in every so often and it's hard to keep it in check.
Edit -- One last thing. Tattooing doesn't hurt too much (IMO) unless you choose an especially sensitive area. If that's the case, then prepare for the pain and accept it as part of your decision

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