A Bike and lots of Revives, and for heal items, I'm a junkie for Moomoo Milk/Water/Soda Pop/Lemonade, because they're better healers at cheaper prices than the various Potions. (A $350 Lemonade heals 80 HP, a $700 Super Potion heals 50 HP. Hm!) Platinum and its dozen Moomoo Milks = awesome.
In Platinum I'm doing an all-Steel run, so for the first time ever pretty much all of the Pokémon on my team who can lose more than 100 HP and still not be in the red. So I'm using a lot of Hyper Potions that I've found in various Routes. When I run out, I'll probably just use Moomoo Milks again instead of buy more. :P Two Moomoo Milks are still cheaper than one Hyper Potion.