Just to clarify, TomG, a normal installation includes lockout switches that prevent the OD from engaging in first or reverse. It won't blow the unit up if you accidentally leave the dash switch on (which is a Good Thing since I forget it all the time). Tom Lynch was asking about a test he was doing, without the lockout switches.
There has been a lot of debate on the "proper" method of shifting the overdrive. Clearly the OD manufacturer's intention was that it could be shifted under full power (somewhere I've got an ad that talks about it "kicking like a mule"). But it's also likely to be somewhat hard on the unit, and I dislike the "bang" when shifting under part throttle, so I generally use the clutch when shifting into OD. Shifting out under power is much smoother though, so I generally don't bother with the clutch for that.
Not really too much else to do, other than keep it full of oil and the oil changed per schedule. Again there is lots of debate over the "best" oil to use; with the factory recommending motor oil in earlier units and then changing to recommending 90 weight GL4 around 1960 or so (without any apparent change in the overdrive itself). I've been using Redline MT-90 for a few years now, and so far it gets my vote for "best". Both the overdrive and main gearbox seem to work better with it than anything else I've tried (and it is a GL4 oil as recommended by Triumph). However it's not cheap; and I haven't taken a look at the thrust washers in the main box (a known weak spot) to see how they fare with it.
IMO, an economical "almost as good" alternative is 20W50 Valvoline VR1 (conventional not synthetic) Racing Motor Oil. British Frame and Engine did a long term test some years ago using that oil, and reported that after 5 years of normal use, the gearbox and OD looked "like new" inside. I used it myself for many years and it worked well enough. The Redline just gives smoother, more positive shifts, IMO.