If you missed it, you might want to take a look at Things I wish I'd known as a newbie perfumista, part 1 for an introduction to today's subject, and many of today's points expand on comments from that post so huge thanks to everyone who took the time to share their experiences.
Don't pay full price if you don't have to. If money is not an issue, buy where you like, but for the rest of us, shopping around is worth the time and effort, and the product you'll get from a reputable discounter (assuming you can find what you want at a reputable discounter) is every bit as good as what you'll get in a department store. You may, however, have to wait another month or two after a fragrance launches before you can find it at a discounter. Want to know if a discounter is reputable? Do a search at the fragrance board at MakeupAlley.
And no, there's nothing wrong with buying testers (the juice is exactly the same, despite any rumors you might hear to the contrary), although if you care about things like outer boxes and caps, make sure they're included in the deal.
Don't buy anything you haven't worn at least 3 times. Rules are meant to be broken, and I break this rule every so often myself. Still, it is worth trying to live up to — first appearances can be deceiving, and it is better to get to know a fragrance, and give serious consideration to whether or not it will work in your collection, before you decide to buy a full bottle. My collection would be much smaller than it is if I'd stuck to this rule more stringently. An even better rule, if you've more patience than I do: don't buy a full bottle until you've used up a sample or decant.
If you're on a budget but really want to try everything, start swapping now. Swapping has its drawbacks, but short of spending an awful lot of money buying samples or full bottles, it is the cheapest way to smell everything on your "to try" list. Before you start swapping, you might want to learn how to get fragrance samples and how to decant perfume.
Niche not always better, expensive is not always better. When I first discovered niche fragrances, I was so enthralled I ignored everything else. It's easy to get carried away. But keep in mind that while there are lots of great niche lines, but there are also lots of overpriced, overhyped niche lines, and there is also some really wonderful stuff happening in the mainstream sector. Keep an open mind and try everything you can.
Don't worry about what everyone else thinks. Your favorite fragrance is a much-maligned little number from Target? Who cares! Wear what makes you happy.
Before you adopt a sample storage system, consider how it will scale as your collection expands. When I started collecting samples, I threw them in a basket. When I wanted to find something, I dumped the basket on the floor and rifled around in the pile. All of that is fine when you have, say, 20 or 30 samples, but when your collection gets too big for the basket, think twice before you just get a bigger basket: ask yourself how you will want your samples organized when you've got 1000 of them, and design your system accordingly. You can read about my search for a sample organization system here and here.