Add new genetics to your line of breeding stock every couple of months or so. You can do this by adding different healthy adults to your current line of healthy breeding stock.  

Raise the new stock for a generation to be sure that they do not have a disease issue before adding them to your current line of stock.

If purchasing new stock (for the purpose of adding new genes) from another farmer, check with that farmer to see where they obtained their stock.  The origin of the stock should be different than the origin of your own stock, otherwise you are simply adding more of (basically) the same genes.

Although inbreeding doesn’t often become an issue with butterfly farmers, adding new genes will help prevent inbreeding issues.