The most important item besides the rifles and ammunition is the footwear you will hunt in. You need to be 100% sure these are comfortable and fit well before you travel. Don't make the mistake I have made, taking new custom boots and ending up incapacitated with massive blisters. Use good socks and wash them before you travel, again not new 'off the rack'. We have recommended the Falke LR socks for years and they have proved totally reliable.
Take something to protect your rifle with on the vehicle and something to clean it with like a bore snake, this you should have with you in your day pack. A sling is often insisted upon so the rifle is carried muzzle up, not pointing at the PH's back. I know many PH's who won't hunt without the client using a sling.
Laundry will be done daily so you don't need an abundance of clothes. Remember early mornings are often freezing mornings so take warm layers you can easily peel off as the day heats up. Again wash your clothes in advance and take items made of good quality cloth. Heat, long walks and running = sweat and can = chaffing, so take some cream also!
Don't forget sunglasses and a good hat, a little first aid kit and also blister packs, suntan cream, insect spray and eye drops as it is very dusty. Malaria pills if travelling to those areas are essential.
A back up camera is a good idea, take a prime and a small spare digital should disaster strike before you get your major trophy!
We are always happy to advise on what items we find have worked and those we found haven't so please do not hesitate to call us if we can help. Have a great trip!