Can't hit irons solidly

I can hardly ever hit the ball solidly with my irons. I just can't consistently hit the ball in the center of the clubhead. It's always either too much to one side, or I just don't get under it solidly enough.

Is there anything I can do to help me hit the ball in the center of the clubhead, or just keep practicing?

Whenever I wanted to tune up my game before I play a round of golf, I practice with my wedges hitting 10 - 50 yards shots.  Focusing on making solid contact, distance control and finally direction.

If you are not hitting the ball solid with your irons practicing with your wedges could help you also.  focus on your take away in direction and height of your take away.  You will know when you make solid contact with your wedges and you can start by hitting hip high, 1/2, 3/4 and finally full shots with your wedges.

I focus on the direction of the toe of my club pointing on the take away around a hoola hoop on plane with your swing plane.

My instructor told me to do the 3 ball drill and it has really helped me.

You just set up two balls at a 90 degree angle to the middle one you plan on hitting.  Set them up just far enough apart so your club face can get through without hitting either of the side balls.  I also put a towel or something behind the ball if I am hitting off mats at the driving range so I will know if I hit behind it.

The good news and the bad news is that it's probably a lot of things that will get better as you get better over time. For me, over the years, I have become a better ball striker with all my clubs as I learned to incorporate numerous tiny things such as better weight shift management and balance, a better understanding of how to use my lower body and free the hands to do the fine tuning work of club face control, adjusting my spine to create a good plane to swing the club head squarly into the ball, and even developing the proper concept of hitting down on the ball and how to use an iron to strike a ball. All these things seem obvious but it takes time to incorporate them into Your swing. Eventually, your swing gets better but it's little thing after little thing that will allow you to strike the ball the way you want to... And then you will still find there is room to get better.