Inspired by this post, I tried using some cardboard from a box of pasta.
It worked pretty well.
I don't think it will last forever, but there is plenty more cardboard produced through normal purchases.
Fold the box flat by open the ends.
Mold the box into a curved shape similar to that of a shoehorn. Insert into shoe, and your foot should slide in to the shoe.
I think a box of macaroni might work best based on the width; something around 3-4 inches is ideal.
lifehacks.stackexchange.com/users/2431/Minnow
In general most items which are reasonable flat and narrow can be used as replacements for a shoehorn.
Items like rulers, a suitable stick, large wooden spoons, or similar items.
If you are in a real pinch, you could use other kitchen utensils, like pie cutter, sausage tongs.
Anything long and flat could do.
On an alternative approach, you could try using your belt.
It is not as sturdy, but usually they are easily found and accessible.
Just insert the belt end into the shoe, and use as normal.
lifehacks.stackexchange.com/users/4490/holroy
After you finish your yogurt (or pint of ice cream), cut the plastic container vertically into a medium-wide strip and trim off the bottom.
Bend the strip lengthwise to fit into the heel of the shoe.
A dollar-bill sized strip of polypropylene (PP) will lay flat on a closet shelf patiently waiting for your next application.
It will fit into your hip pocket, wallet, or bag.
It works wet or dry, and can be used for other things that a bobby pin can't.
It's one of a few great MacGyvers* to have with you.
*MacGyver is a different kind of hero: he doesn't use weapons, he's afraid of heights, and he's an expert at making complicated machines out of ordinary things quickly.
http://www.tv.com/shows/macgyver/
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I use about 6-8 Business Cards -- right size and shoes slide on.
Generally have that many in my wallet...
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A plastic bag can be used.
Just fold it lengthwise till it's about the inside width of the heel, place it in position while holding one end and slide foot into shoe.
It's easy and it does work.
Cheers.
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I use a piece of letter paper, fold it in half a couple times to the shape of a shoe horn and it works very well.
lifehacks.stackexchange.com/users/17164/paul
I would cut a plastic bottle of the correct size / shape
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B"HHey, I'm new here and have been using a Credit Card as a shoe-horn for years.
I just came online to see where I can share the info... and did a search - and this thread came up.
A credit card works like a GEM as a shoe horn.
Just make sure to position in on the inside corner of your shoe/foot.
In lieu of a CC - for not-so-tight shoes - use a regular business card.
Best, - Chayim
lifehacks.stackexchange.com/users/23743/Chayim B. Alevsky
I have resorted to pens for this often in the past and found they worked about as well as fingers, minus the pain.
Try carrying a pen or having some pens by your door.
You can also try two pens, or add a finger next to the pen if needed.
lifehacks.stackexchange.com/users/25212/Tsokei Fascinth
I can’t believe no one has mentioned using a toilet paper roll.
After you finish that next roll of toilet paper, don’t throw away the cardboard roll,;..
Just slice it in half and now you have two shoe horns!!
Brilliant
lifehacks.stackexchange.com/users/26043/user26043
The title says it all... I need something to use instead of a shoehorn.  I tried a spoon but this does not work well.
What can replace a shoehorn?
lifehacks.stackexchange.com
<substitute-tool><shoes>
lifehacks.stackexchange.com/questions/9304
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There are a whole range of items that you can use instead of a shoehorn. Cardboard, such as from food packaging or old toilet rolls are a good choice. Similarly, you could consider using business cards or a credit card. Other options include cutting up a plastic bottle or yoghurt pot, using folded up letter paper or using a plastic bag. The options are to use cardboard from a pasta box, any flat narrow object such as a ruler or your belt, a strip of plastic cut from a yoghurt container or bottle, a stack of business cards, a plastic bag, a folded sheet of paper, a business card or a toilet roll cut in half.
Use the cardboard from a pasta box, folded over. Use any flat narrow item such as a ruler or a wooden spoon or even your belt. Use the plastic from a yoghurt or ice-cream container, cut into the right shape. Use 6-8 business cards one on top of the other. Use a plastic bag, folded lengthwise. Use a sheet of letter paper, folded up in the shape of a shoe horn. Use a strip of plastic cut from a plastic bottle. Use a credit card or a business card. Use a pen - or two if needed. Use a toilet paper roll sliced in half.