I plant mine in an 'X' grid.
I.e:
X X X
X X
X X X
(Hope the above grid makes sense). This helps the sun reach each tree as it moves throughout the day.
Any taller rootstock trees to the 'rear/outside row', not in center, so that all benefit from the sun. To aid pollination, I keep all the trees together by type, eg: apples in a row, plums in a row, pears in a row etc etc.
Planting distances as per your rootstock, and I use a good mix of well rotted compost, the earth from the hole and (for my heavy soil), sand.
After planting keep the grass a good foot away from the tree base and water copiously. Don't forget to use a stake and proper tree tie per tree, and a tree guard to stop the bark being nibbled.