To be brutally honest the service you will receive in most places in Namibia leaves a fair deal to be desired, and often fall well below internationally accepted standards. Often poor service is a result of poor training and also a complete misunderstanding of what is actually expected (imagine trying to be a waiter if you have never actually eaten in a restaurant). However a 10% tip is customary in restaurants - and if the waiter has smiled and attempted to communicate this should readily be increased.
Tipping in bars is uncommon but appreciated. Porters and anyone who packs & carries your bags at a supermarket should be rewarded with a good solid fistful of loose change.
In Windhoek look for the car guards who will look after your vehicle in most parking lots (this is not a scam!) and is well worth the investment, give them around five dollars per hour.
It's worth remembering that as a wealthy foreigner (and all foreigners are wealthy - if you were genuinely poor you would be back in your home country and not swanning around Namibia on holiday) a few dollars does not mean a whole lot to you but can actually enhance a locals day.
"Here, have a dollar, in fact no brotherman here, have two. Two dollars means a snack for me, but it means a big deal to you.", Arrested Development (from Mr Wendal)