Although it's probably true that much of what we call herb wisdom in cooking is the result of trial and error -- at least, it's been that way for me -- I do have some tips that will help make your spice cabinet a holiday friend instead of a beguiling frienemy.
Fresh to dried - There's a big difference between dried and fresh herbs. You'll read proponents of both, citing specific herbs that will respond better to one type of handling or another. You can count on the fact -- proportions DO vary. Dried herbs contain concentrated, flavor enhanced oils. A good rule of thumb is that you will need one third the volume of dry herbs as you as you will fresh herbs.
Example: A recipe that calls for one teaspoon of dried parsley will require a tablespoon (3 teaspoons) of fresh parsley. There are some possible exceptions, though. If an herb has a pungent aroma, like sage or thyme, you can typically double instead of triple the conversion rate from dried to fresh (or vice versa), and then season to taste after that.
Old herbs and spices - Conventional wisdom was that dried herbs would only retain their flavor for six months or so. This turned out to be a big boon for seasoning manufacturers, but not such good news for cooks forced to buy large jars of herbs and then end up pitching half or more of their contents. The thinking has changed somewhat in recent years: Most food professionals now believe many dried herbs and spices will last at least a year, and sometimes two or three years. Your best bet is to date the herbs you buy, and use the sniff test once they're a year old or more. Pinch older stored herbs to see if they have the aroma you associate with them. If they're losing their "punch," replace them -- or in an emergency, use somewhat more in your recipes, say a quarter to a third more or so.
Preparation - There is a difference between buying ground spices and grinding them yourself. The classic example is nutmeg, which smells amazing when it's fresh-ground but loses aroma quickly. This rule applies to quality cinnamon and other spices as well. If you have a hectic lifestyle and want to produce a great meal, you probably have more to worry about than prepping your own spices, though. Using pre-ground spices and herbs will likely provide enough zest, especially if you're using them in concert with other aromatic ingredients. Grind your own if you can; if not, don't sweat the small stuff.
Cooking times - Lots of old-timey recipes are favorites around the holidays. They may be a family tradition, or just have charming or enticing elements that make them seem perfect for a Norman Rockwell style feast. Don't be fooled. Even though some older recipes call for adding herbs at the beginning, most herbs are best added within the last half-hour of cooking or so. Exceptions are some individual herbs (bay leaf) and herb bags or tied herbs that can be removed later. Very long cooking times typically turn herbs bitter, dark and unappealing.
Dried Herbs - If you're buying dried herbs and spices to use this holiday season, store them in a dark, dry location. Spices sold in cellophane packets (scandalous!) should be transferred to jars with tight fitting lids for long term storage. (Tinted jars are best.) I save my spice jars and refill them as needed.
Fresh Herbs - When you purchase fresh cut herbs from the produce department of your local market, remove a half-inch from the stem end and stand them in a glass of water in your refrigerator. This should net you another few days of useful life. If the volume is more than you need, dry the remainder in a warm (not hot) oven on a cookie sheet (turning often). Another option is to chop them up in water and freeze them into herby ice cubes.
When you harvest fresh herbs, avoid using the stems. Not only are they typically tough, they have concentrated oils that tend to make them bitter. Here's how to harvest leaves from stems: Cut and wash the stems. Pinch the tip end of each stem and run your fingers down to the base using some pressure. This will release individual leaves into the sink or a small bowl. Discard the stems. (One exception is rosemary stems, which make inspired kabob skewers.)
Another option is to purchase live plants. Harvest up to a third of the foliage, and place the plants in a sunny windowsill until spring -- pampering them with water and TLC, of course.
Using herbs and spices can be loads of fun, especially around the holidays when any culinary effort seems to carry with it a backstory and memory making potential. Standing in a warm kitchen filled with the aroma of blended herbs is a treat in itself. The rest, as they say, is gravy. Here's hoping your recipes are delicious and all your little helpers offer to clean the kitchen.