The No. 1 tip for staging a fragrance intervention is to approach the person with kindness. You know what they say -- you catch more flies with honey than vinegar. If the issue is that your mom, your co-worker or your husband tends to overdo it with the scent, it's likely that the offender isn't aware of his or her actions, despite the fact that it's highly annoying for you. If you've built up some substantial irritation about the issue and are afraid you may not be able to say it nicely, plan out what you're going to say ahead of time. It's important to be straightforward about your issue, but try to be as diplomatic as possible. Explain that while you appreciate that your friend wants to smell nice, the amount that she's applying is affecting your enjoyment of your common space. If all else fails, you can always cop out and say you're allergic. Just make sure you don't wear your own perfume to that conversation.
If the issue is that you like fragrance but don't care for the one the person is wearing, the easiest way to deal with that is to gift them with one that you like or recommend. You may want to mention it in an off-hand way before purchasing to see if they have an opinion about the brand. Better yet, tell them you're interested in freshening up your personal scent and see if they want to accompany you to the fragrance counter. Best case scenario is they're open to trying a new scent. Just keep in mind that your person has likely chosen their scent as an extension of his or her own tastes, so if they like sweet but you like woodsy, you may not be able to get them to tone it down to your preference. If that's the case, you may have to come clean and just tell them you don't particularly care for their scent of choice.