Large export with many fields

I need a simplified UX solution for selecting many checkbox items (around 50) for export from within a modal window. The idea is that the customer would like to be able to chose multiple fields from a large database table to export in to a report. I don't want to just list 50 field names in a scrolling modal because it is difficult to remember what has been previously selected. The user would have to constantly scroll up and down to confirm their choices. I also don't want to shove all 40 field names in 3 columns in to a small modal window as it doesn't look very aesthetically pleasing.