If someone came to me with that, I'd recommend either sticky keys, where you can just press a key and the system holds it down, or VoiceOver's "trackpad commander" which basically gives VO control of the trackpad, so the user can swipe left for the previous item, swipe right for the next, that kind of thing. Of course, you then need to turn the trackpad commander off when you type cause any little tap will be sent to VoiceOver and your focus is moved from the text field you were in, to the submit button right as you press Enter for a new line...
There's also the "numpad commander" for external keyboards with a number pad. I like this; you navigate with the 4 and 6 keys, 4 moves back and item, 6 moves forward, and there are keys to skip to the next heading and such. Of course, for hand tremors, I'd really hope one day Apple's voice control and VoiceOver work well together. Right now, it's kinda harder to do if you don't know the name of controls to click on before you get to them with VoiceOver, or if you don't have headphones, the voice output will interfere with the voice input.
There's also the "numpad commander" for external keyboards with a number pad. I like this; you navigate with the 4 and 6 keys, 4 moves back and item, 6 moves forward, and there are keys to skip to the next heading and such. Of course, for hand tremors, I'd really hope one day Apple's voice control and VoiceOver work well together. Right now, it's kinda harder to do if you don't know the name of controls to click on before you get to them with VoiceOver, or if you don't have headphones, the voice output will interfere with the voice input.