If your oven smokes or smells when you preheat it, the cause is almost always old food and grease on the oven floor and walls. Every time you cook, that layer heats up again, and over time it hardens into a crust that's harder to remove.
The holidays are when this matters most. Thanksgiving turkeys and holiday roasts mean long, hot cooking sessions, and a dirty oven can fill the kitchen with smoke right when guests arrive.
A proper oven clean removes the racks to soak and scrub them, then works through the baked-on grease on the floor, walls, and ceiling of the oven, and cleans the door glass on both sides. It's worth doing a week or two before a big cooking day, not the night before.
To keep it cleaner afterward, place a baking sheet on the lower rack when roasting or baking anything that might bubble over, and wipe up spills once the oven has cooled.