Keep it in the box in the same housing where it was supplied, along with the mini-manual you get, and the receipt.
If you didn't keep the box, put in a small plastic bag (like a coin bag) inside a larger envelope that is not so easy to lose.
Or use masking tape to fix it to piece of card, and put that in an envelope.
Write on the envelope what it contains.
Or – hack – use an opened paper clip instead, if you need to remove the SIM.
In both cases, insert the tool carefully at right-angles – not at a slope – so you don't damage any internal component.
Edit : in reply to those who say a paper clip does not work, the first paper clip I tried is exactly the diameter of the supplied tool, and successfully ejects the SIM cradle.
YMMV.
lifehacks.stackexchange.com/users/28188/Weather Vane
If it has a small hole on its rear end, put it on your keyring.
Some designs don't have this, some do.
lifehacks.stackexchange.com/users/30936/Keyringer
If you are using case for your phone - consider dropping it inside when you put the case on.
lifehacks.stackexchange.com/users/30938/Ivan Klimovich
I keep it in my wallet, with the coins.
Not only it doesn't get lost, it's also always available when I suddenly unexpectedly need it.
This may even be a lifehack, because it wasn't meant to be kept in a wallet.
lifehacks.stackexchange.com/users/21176/Headcrab
Disassemble an old speaker if you can find one, and take the magnet off the tweeter or woofer.
Find a nice metal place to stick it to on your desk, then have the magnet keep a hold of the sim ejector.
lifehacks.stackexchange.com/users/30939/James Lingerfelt
Don't worry about losing it; plenty of every day objects can be used as a sim ejector: a safety or sewing pin staple (opened) paperclip drawing pin an earring (pin, not clip on type)
a fine drill bit If you're really desperate, filing a hair clip down with a nail file etc..
I've had to resort to stripping the wire out of a freezer bag tie and tripling it up/
twisting it really tightly before now.
It should also be noted that your chosen object will need to fit in the hole and be slightly loose especially if the hole is in the moving part of the door because as you push the tool in, the door will want to slide in the opposite direction; a tight fit will inhibit this and could result in breaking the tool Most sim ejector tools I've come across are approximately shaped like a sports racket; the hoop section could thus attach to something you typically bring along, like your keys or headphones wire (form a loop in the wire, push it through the racket then back over itself around the outside of the racket, perhaps even the zip pull of your jacket.
If you have a shock proof case for your phone it should also easily sit between the case and the phone
lifehacks.stackexchange.com/users/25171/Caius Jard
What I've done is keep SIM ejector tool in small plastic container I already have.
And keep it in another one where I keep my USB flash drives .
lifehacks.stackexchange.com/users/30931/Hemlata
Personally, I keep it in my wallet - either in the coin purse, or in the cards section.
This allows me access to it when I am on the go as well.
lifehacks.stackexchange.com/users/30945/Chris
Keep it taped to the back of your ID (or another card that you always have with you that doesn't have to be tapped/swiped).
This way it will always be in your wallet/purse.
Bonus tip: If you have two sim cards (because maybe you do business in multiple countries), keep your other sim taped like this as well.
You'll always have them when you need them.
lifehacks.stackexchange.com/users/11647/spacetyper
A SIM ejector tool is very tiny, and it is difficult to keep it safe from losing it or missing it anywhere. How can I keep it safe?
How can I avoid losing or missing a SIM ejector tool?
lifehacks.stackexchange.com
<substitute-tool>
lifehacks.stackexchange.com/questions/23367
lifehacks.stackexchange.com/users/30931/Hemlata
There are many different ways to avoid losing a SIM ejector tool, such as keeping it in a smaller container or an envelope. You could also tape it to the back of your ID or keep it in your wallet. If you use a phone case, you could also keep it in there. The options are to keep it in the housing it came in, put it on your keyring, keep it in your phone case, in your wallet, on a large magnet, in your phone case, in a plastic container or taped to an ID card or other card.
There are many different ways to avoid losing a SIM ejector tool, such as keeping it in a smaller container or an envelope. You could also tape it to the back of your ID or keep it in your wallet. If you use a phone case, you could also keep it in there. Keep it in the box in the same housing where it was supplied. If you didn't keep the box, put in a small plastic bag or plastic continer or use masking tape to fix it to a piece of card.  If it has a small hole on its rear end, put it on your keyring. If you are using a case for your phone you can put it inside when you put the case on or you can keep it in your wallet, with the coins or keep it taped to the back of your ID.
The SIM ejector tool can be kept in the original box, in an envelope, or in a plastic container. Some people keep it in their wallet, in the phone case, taped to their ID card or on a keyring. The simplest way to keep this tool safe is to store it in a larger container, such as the box your phone came in, an envelope or even your wallet. Alternatively, you could fit it to something you always carry around with you, like your ID or your phone case. If the tool has a hole, consider putting it on your keyring.
Keep it in the housing it was supplied in. Put it on your keyring. Keep it in your phone case. Keep it in your wallet. Keep it on the magnet of an old speaker. Keep it in a small plastic container. Keep it in a shock proof case between the phone case and the phone. Tape it to the back of your ID card or another card.
S0: Keep it in the box in the same housing where it was supplied. S1: If you didn't keep the box, put in a small plastic bag or plastic continer or use masking tape to fix it to a piece of card.  If it has a small hole on its rear end, put it on your keyring. If you are using a case for your phone you can put it inside when you put the case on.   You can keep it in your wallet, with the coins. Keep it taped to the back of your ID.
The simplest way to keep it safe is to put it inside a larger marked container, like an envelope or the original box that the phone came in. You could keep it in the coin section of your wallet. You could take it to the back of your ID or another item that you always carry around with you. If you keep your phone in a case, consider storing the SIM removal key in that case. If the tool has a hole in it, you might want to keep it on your keyring.