Ghosting can be annoying — although some may find it aesthetic. But keep in mind that if you have a porous chalkboard, such as one made from slate or vinyl, you may always have a bit of ghosting.
That being said, here are some handy ways to avoid ghosting:
Use the right kind of chalk or chalk markers. You may have to go through a bit of trial and error. Always do a spot test in the top corner. Write a little word, then erase it. If white residue remains, you may want to try another pen or chalk.
Prime your board. Coat the entire board in two layers of chalk: go in vertical strokes, then horizontal. Then use a rag or eraser to wipe the board. Blow the remaining chalk away. There will be a soft white layer, but it will also act as a barrier for more intense ghosting. This is a fantastic method for porous boards.