I don't know which bike you're using or what parts of Africa you're heading for but when I've done the same sort of planning my priority has been to think about the bits that would stop me in my tracks.

So - things like can I fix a puncture in the middle of nowhere (spanners, tyre levers, tubes, patches, pump)

Will the bike start in the morning - good battery, wiring issues, electrical stuff like plugs.

Can I fix the likely result of a small "off" - levers (if they're not protected), a selection of nuts and bolts of different sizes. (if it's a big "off" it might be you that needs fixing more than the bike).

Are there any things specific to my bike that I ought to consider - if I was doing another long trip on a Honda XR600 for example I'd look very carefully at the kickstart area having had one snap off in Mauritania.

There are a whole range of things that give you some sort of warning before they cause problems - you can see tyres wearing out by eye and you can check the brake pads and air filter now and again to see how they're doing, as you can with the chain and sprockets. If you start off with all of those either new or in good condition you're unlikely to be left stranded at the side of the road and can plan replacements for when it's convenient.

You can always get bits sent out to you if something unexpected goes wrong and as others have said DHL seem to be the African couriers of choice. It's not a quick process though (you could be waiting weeks for the "paperwork" to be processed) and it might be a good idea to check out exactly how their system works before you go. Unless it's something specific to the bike though you'll be amazed how resourceful the locals can be and stuff you never thought you'd get can turn up if you just ask around.

Lastly, several times (over a very long period though) I've had bits sent out only for them to be the wrong part when they arrive. That's what comes of trying to describe the part you need to a UK (in my case) dealer over a crackly phone line. Take a parts book or similar (on a USB stick is easiest) and make sure someone you trust has the same book back home. They can act as spares liaison and let you know what's happening.