You may not realise it but an improperly weatherproofed exterior door is a huge energy user. A crack that is even 1/8th of an inch wide on a typical door is the same as having a 36” window open 1 inch wide all winter. Think about looking at all that heat, and the money spent warming your house, going literally out the window the next time you ignore your exterior door.
The first step is making sure that your door is true. Over time your house will settle and shift, this is what usually causes those 1/8th inch cracks in the first place. So not only can this correct the issue on its own in some cases, but it will also save you money on weatherproofing materials.
Metal tension strips should be installed where your door contacts the stops when it is closed. A putty knife can pry those strips out far enough to make a good seal that keeps heat in when the door is closed. A vinyl or aluminum door jamb weatherstrip on the external side of the door edge will also keep heat in.
The bottom of the door can be sealed up with a bristle sweep. You can also attach a door bottom that is a bristle sweep on the interior, and a drip edge on the exterior. When doing this you may have to slightly adjust the height of your door by shaving some off. If you do shave it off slightly be sure to reseal the wood.
When looking at the entirety of the exterior door, fill any cracks around panels with wood filler or caulk. A good coat of paint or a refinishing will complete the look of your now weatherproof exterior door, readying you to stay warm all winter.