Always keep canned tomatoes in the pantry. Particularly tomato paste. It's a great ingredient that can add a depth of flavor to a variety of dishes.
Whole spices really do make a big difference and it's worth knowing how to use them even if you don't make them part of your everyday cooking. When its time to make a special dish, using whole spicessl that you grind fresh will be worth it.
Use acid in your cooking. Citrus, viniger, wine, yogurt, etc will often upgrade a dish.
Parchment paper/aluminum foil is your friend. Minimize your time scrubbing dishes by minimizing the mess you make when baking food.
I have a few recipes that were clearly written or modified by someone using really old ground spices. Like I have a recipe for apple muffins that legitimately calls for two teaspoons of nutmeg.
I do like 4 rubs of a nutmeg on the microplane and it's perfect.