Update parent record with SUM of two related tables

I need to update the parent record with the sum of the related table and wondering what would be the best way to achieve this. Would the best way to add a function to the create & edit forms of the related table so it calculates and updates, or is there a better way of doing this? I'd also like the parent fields to update when a new record is added or edited. Any advise on the best way would be appreciated.