I suppose I should actually start with why you might want to decant perfume. Well, there are any number of possible reasons. Perhaps you want to start swapping (swapping article coming up soon!), or selling on ebay, or perhaps you just want to share fragrances with your friends or split a bottle of something you can't afford to buy full-sized. Or maybe you'd like to have some travel sizes of your favorite scents?
To start with, you'll need some empty bottles, atomizers and/or vials. I personally use 2 sources for empty bottles, pilotvials and madinaonline, but you might want to post a query on one of the fragrance forums to see what suppliers are popular at the moment; I don't order often enough these days to know which suppliers are considered "reputable". If you're planning to start swapping, you'll probably want to have some small glass vials (the "standard" small vial is 1/32 oz), and then an assortment of larger sized bottles. I am partial to 1/8th oz decants — just enough to make up your mind about the fragrance one way or another — but I'm guessing most swappers prefer a slightly larger size, perhaps 1/4 to 1/3 oz. For bottle splits, you might need 1/2 or even 1 oz containers.
You will have to choose between splash, roll-on and spray bottles. Personally, I prefer splash except for travel (see below), largely because they are cheaper than the spray. Some people prefer roll-on, but I do not like the idea of having the fragrance contaminated with every use. I used to remove the roll-on mechanism from any such bottle I received (you can pry it out with a tiny screwdriver, like the kind that are sold to mend eyeglasses), but with many roll-on bottles, that mechanism is what creates a tight seal with the cap; if you remove it, you risk evaporation unless you move the juice to another container.
For travel, 1/3 oz atomizers are perfect. The biggest problem with travel atomizers is that they are prone to leakage. I have been very happy with the 1/3 oz atomizers from pilotvials, but again, you might want to ask around on the boards before you choose.
Glass is generally preferable to plastic. Other supplies that may be helpful: a stainless steel funnel, small plastic pipettes (check lab supply or hobby stores), clean straws (McDonalds!), small clear plastic baggies to hold the vials (I use 2 inch by 3 inch ziploc baggies, which you can buy quite cheaply on ebay) and labels.
A question that pops up frequently is "I've used up a decant, how can I clean the bottle and reuse it", and the answer is quite simple: don't. You can easily enough remove all traces of the fragrance from a glass bottle, but the caps and the spray mechanism, if any, are usually made from plastic, and there is virtually no way to clean them adequately.
How to decant from a splash bottle
If your fragrance is in a splash bottle, you can, of course, simply pour it into the new container. This can be risky depending on the size, but I've done it often enough when I've been in a hurry. A stainless steel funnel will help to avoid spillage, and I also find it helpful to stick the empty bottle into a wedge of clay or styrofoam so that it won't get knocked over while I'm working. If you don't have a funnel, you can fashion one out of tin foil. Avoid plastic funnels, as it is nearly impossible to remove all traces of scent once they've been used.
If you're trying to make a small sample vial from a splash bottle, disposable plastic pipettes are perfect. If you don't have one handy (and who does?) using a clean straw as a siphon usually works quite well. Again, it helps to stabilize the vial in a wedge of clay or styrofoam so that you don't spill.
How to decant from a spray bottle
Start by seeing if the top unscrews easily, but most are crimped on and cannot be removed without breaking the spray mechanism. In that case, the best method is usually to spray directly into the new container by holding the nozzle right up to the opening. This usually works well even with small containers, like sample vials, but some atomizers are much better than others. A very strong stream can make things difficult when you're trying to fill smaller containers (most of the fragrance just splashes out again). If you have trouble, again, a stainless steel funnel, or one fashioned out of tin foil, is helpful.
On splitting bottles
For those of us who already have large perfume collections, arranging bottle splits is the perfect way to get something new without having to take on another 100 ml of something you'll never manage to use up during your life time. A few words to the wise: always pre-measure the amount of liquid that fits into each empty container, in other words, don't assume that a 10 ml container only holds 10 ml. For the casual swapper, a difference of 1-2 ml is not a big deal, but it will make all the difference in the world if you're splitting a 50 ml bottle between 5 people and you are left with only a few drops after you've decanted the other 4 portions.