There are no preventative measures except to practise good cultivation.
If the June drop is severe every year and affects yields, the cultivation regime of the tree should be reviewed. Pruning to allow more light into the tree is one aspect that gardeners can try.
A lack of nitrogen is thought to be sometimes involved so attention should be paid to applying a general fertiliser such as Growmore each spring. However, be careful not to over-fertilise as excessive nitrogen can itself lead to fruit losses.
Insufficient soil moisture may contribute to fruit fall. Watering, followed by mulching, in dry spring weather may help.