While shopping in a stationery store, I came across some star-shaped post-it notes in a bright yellow colour. I decided to try them as a budget option for a wall decal and I think they look pretty good. While these don't sit completely flat on the wall, I like the added dimension provided when the edges lift a bit.
So, for a fraction of the price of designer versions, I have my own version of a decal for my boy's room.
If you like the look, all you need are some shaped post-it notes (stars or other shapes such as pink heart shapes). Simply stick them over your wall until you are happy with the amount and arrangement.