Make sure your team is well balanced. You should have one of every type. That way the type advantage will usually be on your side! Here is my team as an example.

Charizard (Fire Flying)
Blastoise (Dark Water)
Venusaur (Grass Poison)
Mewtwo (Psychic)
Dragonite (Dragon Flying)
Raikou (Lightning Ground)

This was only an example. This team though has never lost.