Add more SIM cards
If you need to switch between multiple SIM cards, then you can, a) Keep fiddling with that tiny SIM tray until one day it springs out and is lost down the air vent in your car, b) carry multiple handsets, or c) fit a SIMore adapter.
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In-car mount
If your smartphones spend a lot of time in the car, it's vital that you have a decent in-car mount for it. I've tested a lot of different mounts and the one I keep coming back to is the RAM Mount X-Grip.
Not only does this fit a wide variety of smartphones - it's big enough to accommodate my iPhone 6S Plus in a protective case - but it can be coupled to a wide variety of different mounting devices (such as suction cups and yoke clamps), so you can use it in the car, bike, motorcycle or even a boat.
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A 'Swiss Army Knife' hub for your smartphone
Take a look at is the IOGEAR MediaShair WirelessMedia hub. This device brings a whole raft of features to the table. Not only can you plug an SD card (SD up to 2GB, SDHC up to 32GB, and SDXC up to 2TB), USB flash drive, or even a portable USB hard drive into the MediaShair hub and the built-in Wi-Fi can share the content with up to seven iOS or Android devices, it has a built-in battery that can be used to recharge your devices.
The device also supports pass-through, so you connect the hub to the internet-facing Wi-Fi connection, and then connect the devices to the hub to allow them to have web access. This way you have access to both the internet and your data.
Access to the data on the device from iOS and Android device is handled through a free app.
On top of that, if you're in a hotel that only has Ethernet connection, you can connect the MediaShair hub to the wired connection and share the connection wirelessly.
This doesn't just add extra storage, this is like having a Swiss Army Knife for your iPhone., and for $99, it's up there as a "must have."
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Charging cable
You can never have enough charging cables, and given that they tend to have a pretty rough life, I like to buy something that is going to last a while, which is why I'm now using the almost indestructible MOS Spring Cable, which comes in both Lightning and micro USB format.
I've been testing this cable for a few months now and not only is it still in one piece, it still looks as good as new.
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Smartwatch
The Apple Watch is undoubtedly one of the best smartwatches currently available for iPhone users.
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Smartwatch
For Android users (or iPhone owners who don't fancy the Apple Watch) there's the Pebble Steel Time smartwatch. Unlike many smartwatches that rely on gimmicks, the Time Steel is a solid, well-focused device.
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Add more storage
This one is for iPhone and iPad users. I just use the SanDisk iXpand flash drive which offers me an extra 128GB for photos and media. As an added bonus I can use this as a regular high-speed flash drive on my Mac or PCs.
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Charging while on the go
I use to suffer from battery life anxiety whenever I needed to use my iPhone heavily, but I've overcome that by carrying a small external battery pack with me when I'm out and about. Yes, I know that I could go through all my settings disabling everything, but why carry an iPhone around with me in the first place if it can't do anything?
If you want a small battery pack that you can pop into your pocket, then the Anker PowerCore+ mini is a great choice at only $9.
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In-car charging
I also keep a charger in the car. And since I'm sometimes looked upon to offer others riding with me a charge-up, I've upgraded to the Anker PowerDrive+ 3 car charger. It's only $15, but it has no problem simultaneously charging multiple smartphones and tablets.
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Better sound
Good headphones are an absolute joy to use, and the Bose QuietComfort 35 noise cancelling headphones are some of the best I've ever used. They have a deep, immersive sound, an advanced microphone that works well in a variety of conditions, even when it's windy, amazing noise-canceling technology, and a battery is good for 20 hours of usage.
They're also incredibly robust, and capable of standing up to daily use.
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Better speakers
If you're in a quiet room then the speakers built into the iPhone are just fine, but if you like to enjoy your music when outdoors or in a noisy environment such as a workshop, you need beefier speakers.
A speaker that's been by my side when I'm out and about is the Divoom Voombox-outdoors. This is a splash-proof, dust-proof Bluetooth speaker with a built-in rechargeable battery that's good for up to 12 hours. The 15 watts of output is ideal for playing tunes around a BBQ or when doing some gardening.
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Toughened case
In all my years of owning an iPhone I've never had the misfortune - or expense - of breaking one as a result of dropping it or through water damage. But this isn't good luck, it's down to putting Apple's frankly fragile creation inside a robust case.
My battle-tested case of choice is the Anker Ultra. It's chunky but this has protected my iPhone from bumps, scratches, scrapes, and even rain for many months, and as an added bonus it still looks like new. I've even dropped my iPhone on its face onto concrete several times and even scuffed the front protective layer a bit, but the iPhone is unscathed.
And not only does it add much-needed protection, it makes my iPhone much easier to handle because the rubber is nowhere near as slippery as the metal and glass of the iPhone.
Anker also make protective cases for several Android smartphone models too.
This is the iPhone case that I use daily.