Personally, I think because you are accepting of a lot of options (specific piercings, hair coloring) you have a little bit more credibility when you say, "Please don't do ____ because ____."
My oldest niece is 9 years younger than me and I watched her go round and round on this issue with her parents. She had several piercings that I thought were cool (and told her) and she was always doing something different with her hair. Many people freaked and her mom was always complaining.
Then she wanted a tattoo (this started around 16). Her mom saying, "no, no, no" didn't get anywhere--- she had said the same thing over and over before. But when I said it was a really bad idea to get one that young, that her body was still changing, that if she waited until she was 20 I would pay for the first hour, lol, she took it a little more seriously. I could show her my tattoo and say, "I'm not against tattoos, I just don't want you to make a permanent mistake." And she didn't get a tattoo. She still doesn't have one (that I know of, I know she didn't have one at 20) and now she is 26. I also talked to her about my concerns about eyebrow rings (movement/rejection) and tounge rings (teeth problems) but that didn't stop her.
For my own children, I hope they don't do anything too permanent before they are adults. I've talked and talked to them, though, about how tattoos & piercings are MEDICAL decisions and you don't just let your friend with a potato and a needle perform minor surgery on you! If they are going to get something pierced, I want it to be clean & safe. If that means they get something pierced I don't want them to, but I end up driving them to get it done, so be it. DD got her ears pierced at 9, DS doesn't have anything pierced yet.
DP used to tell kids about his horrible piercing times (he loves them, but he is very infection prone--- he had a nasty transverse lobe issue going on for years) and tattoos--- we have actually seen kids begging their parents, "Do I have to get that done ever?!?! Please don't make me." I think he should charge them. Since DP has, until January, had long dreads and DS has rotated between long hair, shorter hair and a mohawk, I'm pretty sure our kids understand we just don't care about their hair *too* much (it must be clean, that I do care about--- dreads cannot come about by just being gross).