what exercises work for you to avoid back pain?

I have the feeling I've always had a weak back.

Some 4 months ago I started working out regularly. Now I can almost touch the floor with my hands without bending my knees.

While doing this exercise I heard a small crack on my back, nothing serious.

Since then I can lift weights easier than before but my back still hurts a bit. My back doesn't hurt as much after coming home from work, which was normal before.

What else could I do to have a stronger back?