Over the years of me having ferrets, I have noticed there are 3 main types of ferret harness, the Figure Of 8 Harness, the Vest Harness and the H Style harness, they have their differences.

The Figure Of 8 Harness
The figure of 8 harness is my least favourite harness for a few, they're made of poor quality material, only the neck part is tight enough so you may as well be using collar because it'll likely have the same effect, they're almost always the wrong size and ferrets often walk straight through them so just shows how easily escapable they are. All this makes this type of ferret harness http://2.bp.blogspot.com/-jIU7X6rHSl...0109-00388.jpg not suitable for its use therefore it is useless.

Vest Harness
The vest harness which looks like this, http://www.pawsitive.com.mt/images/f...ness%20set.jpg
I will use on larger ferrets, usually males since for the most part, males are larger than females, though you can get some females that are large and some very tiny males. Sometimes these harnesses are suitable for kits but that isn't often, the reason is that they can be easily escaped since they have a limit on how small they can go since they're non adjustable due to them having Velcro instead of a clip or buckle. My ferret Vinnie has a dark blue one and it fits him perfectly so he doesn't escape from it, and because it's very easy to clean, it's the harness I use when taking him for a walk in a muddy area.

H Style Harness
The H style harness is my favourite harness and if put on the ferret properly, is escape proof, it's easy to put on, depending on the material will alter the way it's put on the ferret, of its nylon then it'll have clips to fasten it and if it's leather like the ones I use, it'll have buckles instead if clips.
These can come in different sizes and can be ordered in specific sizes to fit super small or very large ferrets too. They also come in a large range of patterns and designs too, here's two of my jills, Willow and Whisper in their diamanté leather harnesses https://i.ytimg.com/vi/4ikAajSWMCg/maxresdefault.jpg