Tool handles
Good handles for tools make a huge difference. They avoid blisters and make your hand less tired so you can work longer and achieve better results.
The requirements for a good handle are:
- Tightly fastened to the tool so you get good control without worrying of it loosening up
- THe right size for your hand for the job being done
- Distributing force in the right direction
- Not being slippery to avoid accidents

What to think of:
Fastening the handle
Since I got access to a forge I generally use a piece of iron to forge a false tongue (the piece of the tool to protrude into the wood).
If I'd use the tool itself for this I would risk un-harden it by tempering it too much.
I create a fake tounge with about the same dimension and shape as the actual tongue, and preferably a tiny bit smaller. Then I heat it up and press it into the wood. For each heating I get about 6-8 mm further into the wood. Be careful not to heat the piece of iron so much that it become to soft because then it will bend when being pressed into the wood.
There will be some smoke, and possibly fire, but it's no big deal. The charcoal produced will help tighten the fit when eventually you press and hammer the tool into the handle.
Shape
The shape of a good handle makes a stop at some point. If you get tired and slip, there should be some knob stopping the tool from going to far and cause accidents.
Bear in mind that many tools are gripped in several different ways, and all of them should feel natural. Sometimes you use an ax holding it closer to the blade for balance and control, and sometimes you use it from the end of the handle for force.
<Surface
The surface of the handle should not be to smooth. If you got a well polished handle, add some markings, or even some cuts to it. This way your hand will register any change in the position of the handle within the hand and you need to use less of your strenght and energy to hold the tool.
I tend to make rough handles with edges for this purpose.
Some people add sticky wax to their handles for a better grip. I've found that furniture wax is suitable for my tools. It give the handle a protective layer against moist, give the wood a darker tone and a bit more luster, but is not sticky since I tend to get blisters from the skin stretching with sticky surfaces.
Decorations
Feel free to decorate your handles at will. Don't be afraid to over-do it. I normally go for two stripes and my mark. This help me identify my own tools when working with others.