Enjoy the journey.
Do a bit of pre-planning (checking distances between towns/places on the map to not run out of fuel) (in SA there are very few places where there is not a next dorpie within 60km or so - Namibia is a beeeeg place).
Especially in Namibia do not plan to drive your tank to almost empty believing you will get petrol at your next stop - fill up when you can (have not been in Namibia for a couple of years, but smaller places cash seemed preferred for petrol).
Both SA and Namibia - when not in an actual city, try to not drive when dark (hitting a Kudu/Oryx @ >40km/h will do serious damage). If driving in the dark is absolutely unavoidable - slow down (<80km/h).
With all due respect to the Namibian roads department I found their decision on where to use solid white lines somewhat strange (my personal opinion is that a broken white line on a Namibian tar road was a cost saving way of painting a solid white line

...i.e. make very sure
you can see very far ahead before overtaking).
Take a break every couple of hours (do not wait till you feel sleepy - fatigue is like altitude sickness, by the time you have it, you are already making crap decisions and unlikely to then actually do the right thing).
When taking a break, walk around your car, check tyres etc.
Take a 500ml bottle of water and a small lappie/face cloth/piece of old t-shirt - when taking a break or generally needed wet said lappie and scrub some of the bug splatter of the windscreen (wet the lappie - do not pour water over windscreen) - then use windscreen wash/wipe once or twice to get a nice clean windscreen. (this will be especially appreciated when perhaps
needing to drive after dark and vision is already affected by oncoming lights).
Refill your winscreen washer tank when you refill with petrol. (petrol in petrol tank...water in window washer tank..

)
When perhaps driving the dirt roads - take it easy, most of the dirt roads are (were?) better to drive than many SA tar roads meaning you often end up driving faster than you intended and can get caught out by some loose gravel/sand.
If travelling on a dirt road (especially some of the dustier ones), when an oncoming vehicle has passed be aware that you may struggle to see the road/other vehicles (slow down if needed but don't just jump on the brakes and completely stop).
Support local business (don't try to take all your supplies from SA side).
Enjoy the multitude of colours of the landscape.
Enjoy the journey.