i drove in New Zealand and got used to driving on the left side in short order. Where I would tend to mess up was checking to the left first when making turns. Ended up with my head swiveling several times before pulling out.
The checking to the left first when walking across a street is particularly dangerous. I wonder how many people get smacked making that mistake while walking around town in a left side country?
As a friend in New Zealand told me, "In America, you drive on the right side of the road. In New Zealand, we drive on the proper side."