One rule when taking a passport photo is that you aren't allowed to smile. This makes it very hard to look attractive and often they can end up looking like mug shots. The key here then is to remember that not smiling doesn't necessarily mean that you should be frowning or looking miserable, and the intention instead is to just look basically neutral. You can still look basically happy then by trying to smile with your eyes and having a neutral but optimistic look on your face rather than a forced frown or grimace.

Another thing you need to do is to avoid looking too pasty. It's common for your passport photos to appear this way and especially in photo booths which have a habit of having bright flashes. Even in front of a computer however we often sit too close and this results in our skin colour looking drained and pasty. Make sure you sit back then, and try to tan a little perhaps before you go. Photos from too far away or too close are also unacceptable for other reasons and it is important that you take your photo from around 2 metres away, or that it looks like you did.