This is what helped me the most - shortening the distance between the home row and the mouse. The next best help is to use Vim everywhere which reduces the need to reach for the mouse.
I use a leopold 660m which is a 60% keyboard that has arrow keys tucked close.
A short keyboard (no numpad) is preferable too. It prevents having to open your arms for mousing and typing, which is a frequent cause of mousing RSI.