The PokéBall is arguably the most valuable 'item' in Pokémon Life (for gameplay reasons, PokéBalls do not count as items). You need PokéBalls in order to add Pokémon to your team. You should lay down every PokéBall you receive in front of you, and put the Pokémon you capture on top of them.
You start the game with six PokéBalls, and you can receive additional PokéBalls at any Market tile, the Celadon Dept. Store, the Game Corner, from the Event pile, and from the Victory pile. There is no maximum amount of PokéBalls you can have.
When you release a Pokémon, you don't have to discard the PokéBall. Releasing Pokémon to add stronger Pokémon to your party is a powerful tactic to acquire a stronger team.
The Full Restore is an essential item that you can never have enough of, since a safeguard for when your Pokémon are unexpectedly knocked out is always useful. When you use a Full Restore, you may instantly revive one of your knocked out Pokémon. You cannot use Full Restores during battle.
You start the game with five Full Restores, and you can obtain additional Full Restores at any Market tile, the Game Corner, from the Event pile and from the Victory pile.
Let's talk about Bill. You can use Bill after throwing the move dice, increasing its outcome by 2. This can be useful to land on a tile you desire, or simply to move faster on the board. You can also use Bill in conjunction with Abilities to land on a desired tile.
Bill is especially amazing if you use it when moving backwards with Porygon or Skarmory, allowing you to move further backwards. Bill is also useful to throw away when you encounter Lass or Team Rocket.
Professor Oak is essentially a reversed Bill. You can use Professor Oak after throwing the move dice, decreasing its outcome by 1. This can be useful to land on a tile you desire, or simply to move more slowly on the board. You can also use Professor Oak in conjunction with Abilities to land on a desired tile.
A smart time to use Professor Oak is when the outcome of the move dice is 1. You may then decrease the outcome to zero, allowing you to stay put on a tile for an additional turn, also triggering its effect.
The Miracle Stone is a powerful item, allowing you to instantly evolve one of your Pokémon to the next stage. This is obviously always useful, and it has the potential to be amazing. You cannot use a Miracle Stone on Pokémon who require a specific method to evolve (such as Eevee, Scyther and Magikarp).
The value of the Miracle Stone lies in its versability. You can always evolve your Pokémon on the fly, for instance when you really need to use Parasect's ability but your Paras hasn't evolved yet. Upon evolving, your Pokémon is also revived, so using a Miracle Stone can also be a valid method to revive your Pokémon.
Next on the list is the Gust of Wind. This item is similar to Mr. Mime's ability, allowing you to decide the Pokémon the opponent has to use when dueling you. This can give you the tactical edge required to best your rivals.
When an opponent challenges you to a duel, you have to use Gust of Wind immediately (if you wish to use it), before you choose your own Pokémon. If you choose a Pokémon first, and the opponent chooses a Pokémon in response, you can no longer use a Gust of Wind. If the opponent uses a Gust of Wind in response, you may choose to respond to his Gust of Wind with one of your own.
When you challenge an opponent to a duel, he has to choose his Pokémon first. After he has done so, you may choose to use a Gust of Wind to alter his fighting Pokémon. If you decide to use a Gust of Wind, the opponent may respond with a Gust of Wind of his own.
The PlusPower is an item you can use on your Pokémon when you believe that luck and strength alone cannot win a battle. You may use PlusPower on any of your Pokémon at the start of any battle, increasing its Battle Points by 2 for the duration of the battle.
You can no longer use a PlusPower when either trainer has thrown the first battle dice. If a Pokémon under the effects of a PlusPower is knocked out, it won't lose the Battle Point increase. If you revive the Pokémon during the battle, it will still have increased Battle Points.
The Defender is an item you can use on your opponent's Pokémon when you believe that luck and strength alone cannot win a battle. You may use Defender on any of your opponent's Pokémon at the start of any battle, decreasing its Battle Points by 2 for the duration of the battle.
You can no longer use a Defender when either trainer has thrown the first battle dice. If a Pokémon under the effects of a PlusPower is knocked out, it won't lose the Battle Point decrease. If the opponent revives the Pokémon during the battle, it will still have decreased Battle Points.
The Master Ball is by far the most valuable item in the game. It is basically a supercharged PokéBall, which means that all the rules that apply to the PokéBall also apply to the Master Ball.
In addition, the Master Ball has a few other traits. Whenever you are capturing a Pokémon, you may decide the outcome of the capture dice once. If you do so and capture a Pokémon, you have to put the Pokémon on top of the Master Ball. Moreover, Pokémon captured in a Master Ball cannot be released in any way.
Even though an Egg is technically not at item, it is still included in the list. When you receive an Egg, you have to put it on top of one of your PokéBalls, and treat it as a Pokémon.
The Egg will hatch after you have won four battles with the Egg in your party. The Egg will always contain the lowest evolution form of the Pokémon species you received the Egg from.