Try to make your types complement each other if possible — for example, Water and Electricity are good companions, as are Fire and Grass or Steel and Fairy. Take into account the weaknesses of your types, too. If your Psychic-types have weakness to dark, play Fighting types (because most Dark Pokémon have Fighting weakness) to counter Dark-type Pokémon same as most fire have weakness to water so you should use leaf Pokémon to support the fire types. Remember that you can use Pokémon with colorless attack cost in any kind of deck to bulk it up and fill in gaps. They are versatile and often have many helpful effects. Be aware, though, that no Pokémon has weaknesses to colorless type Pokémon.
Which condition will your deck focus on to win the game? How will it best achieve that? What are the ways that your opponent might counter your strategy? Which cards can you use to minimize your weaknesses and maximize your strengths?
For example, a Blastoise/Keldeo-EX Deck from the 2012 City Championships sported 14 Pokémon, 32 trainers, and 14 energy. It all depends on what you’re trying to do.
Basic Pokémon can be knocked out quickly, so be sure you have some stage 1 or 2 cards up your sleeve. Evolve quickly to give your game some substance after the initial wave of weaker Pokémon. Add “Back-up” - multiples of the evolution chain - so that you will still be able to get your stage 1 & 2 Pokémon out. If you just do 1 of each then when the pre-evolution is knocked out you won’t be able to play your strongest Pokémon cards. Plan from start to finish. Most decks have a ‘start’ card like Cleffa or Pichu. cards like these can help you set-up. Have one or two really big 1-hit K. O. ers you can draw for late in play.
Look for synergies in cards. For example, Hydreigon and Darkrai-EX are a great at moving around Pokémon and energies freely. Look for other great combination to use for your advantage.
Remember that you can put up to 4 of any given card in your deck. If your deck relies heavily on a card, you should boost your chances of drawing it by putting several copies of the card in your deck. You should have around 5 or so cards to support and boost up your Pokémon. Any remaining space can be used to combat against frequently used cards or weaknesses, or to refresh your hand/bench.
