I periodically will check the battery cycles and then use the lowest ones first to get them evened out again.

I also try to use the battery with the least charge cycles to even the count out between all batteries.

I use a sticky label on the batteries, where I write the charge times and the number of the battery in order to keep them equally charged. When there is no more room to write in the charge number, I delete that label and put on a new label.