As previous posts have said it`s trial and error.
There is quiet a comprehensive list on the forum, but to be honest Andy McNab would struggle with everything
Bear in mind that you `shouldn`t` be that far from Control/vehicles etc on any given search, so you should have relatively easy access to more cumbersome items and will generally be back and forth to Control as you are tasked various jobs.
Personally I plumped for the following -
Viper vest (well I`m a bloke and need stuff like this ). I also prefer the weight to be on my shoulders as around my waist (plus I`m too fat for the belt to go round )
My water proof trousers go in the large back pocket
Hi Viz search jacket goes in small back pouch
Water (750cl bottle for me) goes in one of the large pouches on the hips (an extra bottle in milder weather is carried). (My K9 isn`t qualified yet so no need to carry any extra for him)- this is essential, you will use it, even if you think you won`t........once you start walking, searching, concentrating then you drink more.
Small 1st aid kit in spare side pouch when no water in it.
Various smaller front and side pouches-
Spare torch batteries- absolutely essential(in a bag). Nothing more worrying than your torch starts to die and you have no batteries!
Compass
Radio and ear piece.
Torch.
Head torch.
Lighter (to check wind direction, not for a fly smoke)
Barrier/crime scene tape- used to mark anything of interest or mark an area as OOB.
ID card
Pens
Notepad (in a small plastic bag)
Disposable camera
Doggy treats and collapsible bowl
& lastly (I think) treats for me- mints or sweets to suck on.
Probably missed something but hope this helps.