Let’s start with water; (your only compulsory luggage if you check the rules!)
A hydration pack of some description is essential, whilst a hip flask full of something tasty might wet your whistle, it wont actually keep you hydrated. So your choice but Nathan Race vests are a popular choice. Osprey is also a good one, this is a super pack. Camelbak are another perennial favourite.
Kit designed for Adventure Racing is your best bet here; fitted to the body to minimise bounce and designed around the bladder for optimum hydration. You need a few litres for essentials and ideally, access to pockets on your front.
It needs to fit well and be comfortable, and this will vary a lot with your body shape - ladies do look into a female specific cut - so get out and try a plenty before you buy one – a 1000km is a bloody long way with something rubbing – so guess what? Practise with it.
Where to put it all? On the horse? On you?
On you first
Everything in your hydration pack will be accessible at all times and still with you if you and your horse part company. Ah yes, the issue of parting company. Often a deciding factor in where and what you might choose to have on you or on your horse. Bit of a conundrum this one, you don't want your horse to bugger off with all your stuff, even if the horse is recovered, your saddle bag might have gone flying or got half-inched by an enterprising herder. So, you might decide that you want your stuff on you, in a ruck sack or something similar. BUT- think carefully, it’s a long way, it’s bulky, it's heavy, it’s hot and it pretty much always moves – personally I don’t recommend rucksacks. Just think of it as an additional incentive not to fall off. A hydration pack, however, is fine.
I recommend something like a photographers waistcoat. Loads of pockets on your front for just about everything, much easier to access than stuff on your back, possibly even while mounted. Vet card, sunscreen, hip flask in there!
On the horse
You'll have a small swell bag attached to the front of saddle: it has about 1 litre capacity, great for the nick-nacks you need to access constantly: chapstick, rescue remedy, small snacks, sunscreen, but won't stretch to your spare knickers and sleeping bag.
Uniform Saddle Bag behind the saddle much less accessible, assuming you ride facing forward. Although pretty resilient these bags will not withstand being filled to the gunnels and the zip therefore put under so much pressure it bursts at the first buck spilling your wares all over the steppe (littering is not allowed). You also ideally want your stuff to stay dry. So putting things in dry bags and compressing them is the first step to sensible packing.
BIG NOTE ....think carefully about how you pack the saddle bag;
Squashy stuff needs to be in the middle and harder, pokey, pointing stuff in the ends, otherwise you risk rubbing the horse's back. This is the number one issue herders raise with the Chief post-Derby and as much as we realise you are not deliberately hurting the horses, we do see laziness/desperation creeping in after 6 days of packing your life into that bag. If it doesn't sit properly, and cannot be lashed tightly to the saddle to clear the horse's spine, START AGAIN. No excuses. You'll lose half an hour. You'll save the horse from a nasty rub in a country where wound care is not easily available. And the vets will penalise you for rubs.
You will need to think about where you might want to lash an extra layer or some waterproofs (not forgetting the something you'll need for the lashing - leather thong, bungees, laces, gaffer tape) – it will get colder towards the end of the day and you’ll be tired so you might just want something extra on for the last leg of the day. Try and keep things accessible so you don't have to fiddle with stuff whilst mounted. If that means unpacking your saddle bag and re-packing at your final urtuu changeover of the day, DO IT.
If you are definitely going to ride with someone else the whole way you could look at duplication of things like medical kits (the creams, pills and powders actually take up loads of room and are quite weighty) – but you need to be sure you’re not going to be bloody sick of each other by Horse Station 9 and then trying to divvy up the drugs. Remember the medics are there for EMERGENCIES not your day-to-day management, I really don't want to see them dispensing paracetamol and rehydration salts to you, you/your 'team' must plan to be self-sufficient.
Have a double check of the what to pack and rider kit and saddles sections of your handbook before you start planning your packing so you know you're on the right track. The weight limits are also relevant here.