Cold temperatures will mute the flavors in food, limiting the range of what we taste and sometimes making them taste bitter. A gazpacho that seemed perfectly balanced right off the stove will taste bland and flavorless once it has cooled down. Desserts will likewise taste flat and significantly less sweet. (By the way, this is also why coffee tastes more bitter after it cools and why warm soda tastes too sweet.)
Salt is one of your biggest allies! One of its great properties is that it can tone down the bitterness in chilled foods and boost the other flavors. Vanilla does the same thing for sweet dishes, though a pinch of salt also helps. Lemon juice will also brighten the flavors in both savory and sweet dishes.
So you don't accidentally over-season a dish, the best thing to do is to season the dish normally while you're cooking and then season it again once it has cooled down. Taste the chilled dish before you adjust the seasonings and after to be sure you've added enough.
If you're making something like a cobbler or a frittata where you can't necessarily adjust the seasoning once it's chilled, the best thing to do is use your common sense. If the recipe is intended to be served cold, you can probably trust that the flavors will be correct. If it's meant to be served hot but you've decided to serve it cold, add an extra teaspoon or two of salt (for savory dishes) or vanilla (for the sweet ones).
Do you have any other tips for making great-tasting cold dishes?
Related: How Cold Should Beer Be Served?