Teak oil or similar on a regular basis would be a good idea; Danish if you want it to develop a bit of a shine. For serious preservation, one of the systems from Dulux, Cuprinol or whoever ought to be a better bet, but they're all made of nasty poisonous stuff, so get some gloves and overalls, goggles as well.
I wouldn't worry too much about shakes, mostly because if it wants to crack, it probably will in the end no matter what you do. Douggie Fir is good stuff, durable and stable, so that shouldn't be as much of a problem as it would be with other softwoods, especially if you keep it out of direct sunlight.
It's a nice piece of work, definitely an adornment to your doorstep!