Hey clarki, can I clarify that you are looking for information on twitching jigs rather than jigs you would float for, because this underlying assumption makes a big difference.
A lot of the info provided above is great stuff, however, I think it is more suited to conventional jigs you would drift under a float for chum or steel. As you are aware, Coho have almost no interest in this type of presentation and are looking for big bold, in their face movement that will piss them off or spark their curiosity.
This is why curly tails work so well and I continue to use them today, particularly because they are cheap, easy and fast to make.
I never bother using shlappen, because I am not trying to bulk up the jig to get a certain profile, which is irrelevant when you have this demonic thing twitching up and down at a high rate of speed.
The main criteria I consider in twitching jig construction are as follows:
Jig weight - heavier weights for casting distance, penetrating quickly down into deeper water or getting down in faster flowing water. The jig weight will also depends on rod length, sensitivity and action; its tough to feel a 1/8, 1/4 jig or even 5/16 on a long slow action rod. Lighter jigs are better in shallower and slower water or if you need to hold the jig in a particularly depth horizon for a longer period of time. Also the fish are sometimes sensitive to amplitude and frequency, which you can vary on the way you twitch, but will also be influenced by the weight and the materials you use to tie the jig with.
Materials - my goal is to achieve maximum movement, not create a certain profile. I find marabou and rabbit create the best creating movement. The dear hair in the bucktail jigs will move, but not as fluid as marabou or rabbit tail. Marabou is the most fluid of the materials I use, however it collapses the most and is the most streamlined material. This in off itself is not a problem with respect to volume or size, but it does allow the jig to sink faster compared to a bulkier material like rabbit that appears to have more friction and is therefore held up in the water column a little longer. So for any given jig head weight, such as a 3/8 jig head, a maribou jig will sink faster than rabbit, it will penetrated deeper water more quickly and will generally require a faster twitching frequency and have a higher amplitude. However, a similar weighted jig in full rabbit, i.e. with a long rabbit tail (zonker) and a cross cut rabbit body will sink slower and you can fish slightly shallower water and with a slower twitch speed. while still keeping the longer casting distance of the 3/8 jig head. Obviously you can mix materials to fine tune the sink rate.
Tail - try to make as long a tail as possible because that increases the movement as it flutters down. However, the longer the tail the greater the chance it will wrap around the hook. When using rabbit for my tails, if it is the usual narrow stuff, I keep the tail under 0.5 to 0.75 inches (12.5 to 19 mm) from the bend, however if I am using the wider magnum bunny then I often go longer.
Flash - because it is all about movement and attracting attention, always add a decent amount of flash to the jig.
The best combination I have found, to make cost effective and easy (i.e. fast) jigs is to tie, is a zonker rabbit tail and body wrapped with long hair UV chenielle. The tail provides plenty of movement and the chenille provides movement and lots of flash. I can tie one of these jigs in about 2-3 minutes and the chaneille goes a log way and is of a very uniform quality.
HTH