Well, I personally never go anywhere without the following:
Items Pocket (Or the "Loot" pocket as I sometimes like to refer it too).
Max Repel x 30
Escape Rope x 30
Red, Blue, Yellow and Green Shards.
Honey x 10 (For when I can be bothered to smear some trees).
All of the other Useful Stuff/Valuable Items/Rubbish I've accumulated over throughout the game.
Hyper Potion x 30
Ethers, Max Ethers, Elixirs and Max Elixirs (Shame there aren't many of these in each game).
Antidotes, Parlyz Heal, Awakenings, Burn Heals and Ice Heals x 20 of each
Revive x 20
30 x Ultra Balls
Every other kind of Poke Ball x 5 each.
Every kind of mail x 1 Each. (Does anyone actually use these?)
So yeah, all of the usual stuff.