To overcome this we have attached a line (6mm -see picture below) at the top of the mainsail - going down in a straight line to the second reef point (when the sail is up). Now, when we drop the sail and it does not quite come down all the we have easy access to this line which by then should be abou t at the mast base and can be used to not only pull down the rest of the mainsail but also to secure it to the mast base to avoid the wind from blowing it up and out of the boom bag.
Alternatively, if one does not want to go forward at all, one can instead of fastening one end to the second reef point, lead the line all the way down to the mast base and via blocks back to the cockpit. This way you can pull it down from the security of the cockpit. However, be aware that you will get quite some line length with this configuration and there is more chance of it flying off and getting tangled with the sail or other lines if you do not have somebody bringing it in quickly as you lower the sail.