We get asked about this a lot. The common mistake is to hang too high. There are a few things to consider to get it right:
1. What is the height of the eye height of most viewers?
2. Does the work need protected from knocks?
3. Do you have a situation that is unusual?
In this picture, the work is a small work in an intimate seating area. It is likely people will admire this work for a sitting position. So put it at eye height when seated. Don't worry that everyone's eye height differs depending on how tall they are (actually it doesn't differ much apart from extremes) just put it about your eye height. Which bit do I put at eye height you ask? When it has an horizon in it then put that at eye height. If it has a person in it - put their eyes at eye height. If it has a point of interest in it then put that at eye height.
If you are hanging work in a space where people will be mostly standing when they see the work then use standing eye height (avg in NZ is 150cm). You will need to compromise when you are hanging work in a space where people both sit and stand. Pick the one that is the most common and use that as your guide. Works above beds are generally seen when you walk into the room so they should use standing eye height as the guide.
Works above couches or behind desks might need to be higher to avoid people's heads or arms hitting them - raise them 200 to 250mm above the back of the couch. Works behind desks might need to be higher to avoid being knocked by chairs as people use the desk.
There are some situations that are unusual; works of unusual proportions for example. Consider the photo at the top again - if the work was big the guideline of eye height might result in it being hung too low. Tall narrow works might also have to go higher. If you are hanging multiple pieces vertically you might feel best if the middle one is aligned with eye height and the ones above and below are set out accordingly. If you have a lot of works (good on you!) you will just have accept that some will be higher and lower than optimal. They will still look great! Very high walls will also need some adjustment but resist the urge to centre a piece - it could end up too high.
Don't be a slave to this guide. Use it as a starting point and work from there. To save yourself multiple holes in the wall get some low tack tape and make a pattern of the work with newspaper. Tape the pattern up to see what works.