Planning this trip was greatly helped by websites such as trip advisor and Lonely Planet thorntree forum. The best guide book is the Bradt Guide to Ethiopia(5th edition) , their website has an updated section with changes, alterations and helpful posts.
Flying Ethiopian Airlines on 3 domestic flights and Addis Ababa-Rome-London and joining their frequent flyer programme Sheeba Miles gives an increased weight allowance for the flights.
My other hopefully helpful tips pre-departure are;
Take a digital photo of your luggage( in case it is missing/delayed an image in the digital camera wont be lost in the translation when overseas.)
Copies of flight ticket,passport,insurance , in main and hand luggage, also scan them in to your emails,
Register your trip details on line with the Foriegn Office.