As has been discussed since launch, the fingerprint sensor on the Galaxy S8 and S8+ is tough to use consistently. But that's not just because of its positioning — part of the issue is how it sits nearly flush to the back glass with very little separation from the camera lens. Combine these issues, and it can be really hard to distinguish where exactly the fingerprint sensor is without sliding your finger around a bit.
The fix for many is to switch to primarily using iris scanning or Smart Lock to limit the number of times per day you're reaching up to the sensor, but I have another fix: using a slim case to give the fingerprint sensor some physical separation. Now I'm not the biggest fan of cases, and usually only pop one on when I'm traveling or expect the phone to get bumped around a bit. But with the Galaxy S8, the case provides more than just protection — it's a fantastic way to make the fingerprint sensor usable.
I've been using both the Elago "minimalistic" case, which set me back $9, and the Olixar FlexiShield case, which is $10. Both aim to be extremely thin to only provide resistance to scratches rather than bumps or drops, and that's perfectly fine for me considering I really dislike bulky cases. But no matter how thin they are, they add much-needed barriers to the camera and fingerprint sensor, separating the two so you always know that you're finding the fingerprint sensor back there.
A case doesn't completely fix the problem, but makes it a whole lot better.
In using both of these cases (though I prefer the Elago case overall), I have a dramatically higher success rate with the fingerprint sensor and barely any accidental touches of the camera lens. Having this well-defined area to reach for with my index finger doesn't make the fingerprint sensor perfect by any measure — it's still far too high to reach comfortably — but at least keeps the experience from getting frustrating on a daily basis. It's all about having some consistency in using the phone.
When you're looking for a thin case that can help with the fingerprint sensor in this way, be sure to find one that actually has material between the camera lens and fingerprint sensor. Many Galaxy S8 cases offer one big cutout on the back for the camera lens, fingerprint sensor and heart rate sensor, meaning you won't get the full benefit of the case. Product images will give you a good idea of just how thick the case is and how much physical separation they provide between the camera and fingerprint sensor.