Two words: BUBBLE WRAP. We can’t recommend it highly enough, plus any extra padding you can include if the item is fragile. A solid cardboard backing is also a great idea if the item can be easily bent (e.g. a print).
A good rule of thumb is packing it so it could be dropped from a 1 metre height without breaking (packages can be subjected to a little rough treatment along the way). Because we can only give you a full refund if the item arrives in a re-sellable condition, it’s worth swaddling it – and swaddling it good! – before sending. We’d also suggest you use a trackable service, too.