A good way to improve your volleys and overheads is to imagine your opponents' side of the court as a tabletop. When hitting your volleys, you should try to hit the ball in such a way that it bounces once on the tabletop and then the second bounce goes off the court. Many courts today are multi-colored. For example, the inside of the court is dark green and the outside of the court is light green. Using this as a example, you would hit your volleys where they bounced from the dark green to light green, not dark green to dark green.
When hitting overheads, you would think first bounce on the table and then hit the fence, either the back fence or the side fence. Using the color example, you would think dark green then fence.
Don't forget that you can hit the light green area and the fence by hitting angles off the sides of the table, (over the alleys), and not always off the end of the table (across the baseline).
This will make you a much more competitive player. This will allow your volleys and overheads to have more depth, placement, penetration and angle. Think "tabletop" when hitting your volleys and overheads.
Good luck on the courts!