Not done Kili, but have done a few higher altitude trips in remote places, highest being Mera peak - 6500m approx. Nothing can prepare you for the altitude really, so just ensure everything else that you can anticipate, is sorted. This means training hard in the run up. If you can run 10k in 50ish or less, you should be better than most fitness wise. Get plenty of hikes done wearing the same kit you will be wearing on Kili. Tipping up in brand new boots/gear is the easiest way to end the trip early.
Read properly into HAPE and HACE and what the symptoms are. Be responsible for yourself and don't expect someone else to spot that you are seriously unwell. Also read into Diamox. I don't take it, but a lot swear by it. Its a drug that can help with altitude sickness but it will make you pee a lot.
Ensure your personal hygiene is spot on. Plenty of anti bac hand wash too, although this is *NOT* a substitute for washing your hands with soapy water. Ensure that your porters are boiling the water prior to you drinking it. If in doubt, use some drops for the water. You can buy these at gooutdoors, cotswold, amazon etc.
As others have mentioned, if you do have the opportunity to try altitude first, for example at a Ski resort, do so. Ensure that your trip allows plenty of time for acclimatisation in any case.
Last but not least, enjoy yourself. I would rather go to the Himalaya then spend all that time on a faeces infested mountain, but to each their own!