3. Close the letterplate, keyhole and cat flap
These three are small, constant and easy to forget. A standard letterplate is an uncovered slot roughly 10 inches by 1.5 inches sitting at knee height on the coldest side of the house, and on a windy day it acts like a scoop.
An internal letterplate cover with a double brush costs about $10, CA$15 or £8 and screws on from the inside in ten minutes. Buy the type with two brush rows rather than one flap, because a single flap rattles and lifts in gusts. Measure the cut-out, not the external plate, before you order — the external plate is always wider.
Keyholes on older rim locks are worth a $3 escutcheon with a swinging cover. It sounds trivial, and on its own it is, but a rim-lock keyhole on an exposed front door leaks a surprising amount of air for the size of it.
One warning that applies to all three, and to everything else in this list: seal the accidental holes, never the deliberate ones. Trickle vents in window frames, airbricks in external walls, the vent serving a gas appliance and the extractor in a bathroom or kitchen are there to move moisture and combustion gases out of the house. Blocking them is how a draughty but dry home becomes an airtight one with condensation running down the windows and mould in the corners. If a gap was designed in, leave it.
Cat flaps are the awkward one. A cheap single-flap model is essentially a permanently ajar window. If you have one, fit a magnetic or microchip flap with a proper rubber seal, or at minimum add a second internal flap. If the cat has died or moved out and the hole is still there, cut a plywood panel to fit the opening, seal the edges with frame sealant and screw it in — a ten-dollar job that removes a permanent hole in your envelope.