When using shellfish for ceviche, first blanch until medium, then marinate in citrus. Blanching shellfish will prevent it from being tough and overcooked on the outside while the inside remains raw, instead keeping the fish tender and consistent all the way through.
When making salsas, add herbs like cilantro and oregano right before serving. Adding herbs in this way keeps them pungent and crisp. If added to early, citrus in the salsa will cook the herbs causing them to discolor and create off flavorings.
Use potatoes to thicken soups instead of flour (roux). This is especially helpen for those with gluten allergies. If the soup or sauce is going to be puréed, then you can add shredded or diced potato. If not, cook and purée potatoes, then whisk into your soup base like you would for a roux.
When planning a dinner for guests, do prep like blanching vegetables ahead of time, so that all you need to do is warm them up.
Save juice from your fresh made salsas (ie: pico de gallo) to use in soups, ceviches, sauces, or Bloody Mary's.