Add rounded corners to an element and set the corner radius to a low value.
You might not want noticeably rounded corners in your interface, but adding just a small amount of corner radius to elements like form fields can soften their appearance, by making sure the corners are not sharp, without making it seem like they've got rounded corners. The first example below doesn't have rounded corners, and the second example does.
This looks a bit more natural than sharp corners, because most things in life don't have sharp corners.
You only need a small amount for this effect to work. Even if someone doesn't notice the rounded corners, it'll have an effect.
You might want to apply this to every sharp corner in the interface, to avoid having some corners sharp, and some corners slightly rounded.