I noticed my right side was slightly bigger than my left about 18 months ago. First thing I did was start using dumbells more, which is probably the first thing you should be doing.
Also, when I thought about it for a bit, I realised that:
a) I always started any one-arm exercises with my right side first.
b) Whenever I was adding/removing plate's/db's, I was using my right hand to pick them up, move them, etc.
So I also switched it around so I always trained my left side first and I made sure that I was using my left hand to pick up and move stuff.
I'm sure some people will say that little things like that don't make any difference, but I felt it did, and at the end of the day, it doesn't hurt to try.