Get the height right...
The centre/focal point of the artwork should be approximately eye level, (admittedly not an easy calculation if like myself and my husband there's almost a foot difference in height); as a rough guide 150-160cm off the ground should be ideal. You do not want to be craning your neck whilst admiring your artwork.
Get the scale correct...
When faced with a blank wall void of any furniture, the larger the piece of art the better, so go wild!
When hanging a diptych or triptych the ideal spacing between pieces would be 7-10cm.

This spacing can also be applied when hanging multiple pieces of artwork, as the repetition of will produce a sense of cohesion. When creating this gallery wall of artwork, each piece can be treated individually, they do not need to be part of the same series, theme or style. Framing them in the same or similar frames will unit them, if that's what you wish.
Hanging artwork above furniture...
The artwork should not be bigger than the furniture it sits above, aim for roughly 2/3 of the size of the furniture (this can be made up of more than one piece, i.e. a triptych) and hang the piece approximately 15-20cm from the top of the furniture.
Be conscious of the weight of the artwork...
An obvious point but an important one nevertheless and one to consider when selecting fixings, along with the type of wall it will be mounted on, i.e. brick verses plasterboard. Canvas paintings are a great lightweight option.
An alternative option to mounting artwork onto the wall is to lean the artwork, which delivers a contemporary look whilst allowing flexibility. This is perfect for those who are renting or simply like to rearrange their decor or to add to their collection regularly.
This technique can be achieved by leaning the artwork on top of furniture, against walls or through the use of picture shelves.
The use of picture shelves within the context of a gallery wall would allow you to rotate or update the wall with ease.
Framing...
If your style of decor is eclectic then you can have a bit more fun with the framing and focus purely on what compliments the artwork. If not however, then you will need to take into consideration the rest of the decor also. The use of a glass floating frame is quite a simple and effective way to present your artwork, that wouldn't conflict too much with existing colour schemes.
Vary how your artwork is displayed...
Let the space dictate how you display the artwork. A collection of smaller more personal pieces that evoke memories would be more suited to a living room or study, whereas a large eye catching abstract painting would sit beautifully in the dining room, just waiting to spark conversation! By doing this it will feel more organic and less curated.