Coming from a person who uses metric, know your conversions well.
Careful when using margarine in place of butter, as they have different things in them. For example, margarine has a lot of liquid in it.
Butter is also usually "creamier" and "saltier".
When deep-frying, or in fact, generally cooking with oil, make sure your hands are dry. Common sense, but hot oil splatters like madness when water touches it.
If a recipe calls for buttermilk and you don't have 'em, mix a cup of milk with a tablespoon of vinegar.