There are some things that may be considered by some to be cardinal sins and in most cases it's how i feel about them as well. However the keyword phrases here are "in moderation" and "when in a pinch".
One of those sins is to use an image of a certain size in a situation requiring a bigger size by resizing it and in the process make the image blurry. However what can you do in a case where you have no other choice than to use said picture? It doesn't matter if it's too small or anything, there is a reason that you just have to use that specific one and you can't get it bigger. Well there is a cheat that you can do to try and hide the blurriness. Again this works if the image is slightly blurry, CSI magic is just that; magic in a TV series. The "cheat" isn't about making the image sharper as there is already a sharpening filter in Photoshop (which sometimes introduces sharpening artifacts). Instead it is to make the image look as if it is sharp enough or more specifically trick the eye into seeing more resolution and details than it really has.
So what is this semi-solution to blurry image sin? Noise filter. No wait, don’t hang me yet, I’m not talking about uncontrollable amounts of noise and yes; it’s not a silver bullet, it doesn’t always work nor does it have a standard amount of noise that works. It’s really easy to pull it off in Photoshop though, you set the noise filter in monochrome as you want to add false detail, not color blotches. Then you play around with the noise amount slider to find the value that works for your image, starting from the very left to just add a tiny bit amount of noise making it look more like fine film grain than noise. For my taste it's the point where the noise barely starts to get visible. Oh and btw it works better with photos that are in monochrome. (And that means any colour, not just black and white.)