Open the outer flap from outside and examine the corners where the hinge bar connects to the outer edges.
1.1 Look to see if there are springs, in many cases those springs can have rusted away or come loose.
1.2 Some use magnets which can also rust, fall out or get set back.
1.3 Check the bristles around the mouth looking for long or unattached strands as they effect the integrity, allowing wind to enter.
2.1 The flap can be made stable using one or more coins glued to the inside back of the outer flap. Use a good epoxy resin on the reverse and I like to glue around the edges to stop letters catching on it. Any large, flat old coin will do but I've used the 1960's '100 Spanish Peseta' coin.
2.2 Clean magnets with an emery-board to ensure good contact. Simple replacement magnets can be cut from fridge magnets and glued in place. Pad behind them to make sure they are flush and make contact with the receiving surface.
2.3 Ensure brushes are secured and not leaning out. Trim long/pulled bristles. Glue their holder in place if it has come detached. Sometimes removal of the brushes altogether is enough as they can be poorly designed or warped by weather.
3.1 Springs are hard to replace and regular, light lubrication with vaseline once a year keeps them dry and clean.
3.2 Magnets can also be lightly oiled retard corrosion but not so much they loose their grip.
3.3 There is little you can do to prevent bristles warping from weather and use, if you known any please feel free to comment.
[better photographs to follow]