pin very plump and juicy prunes on that side of the tree. or soak over ripe plums in water overnight and water the tree with that water. works a treat for the tree! good luck!
They do tend to do that. Has it already flowered? If so, prune the branches with faded flowers on the widest side hard back to even it up. If it's a later, summer flowering one prune it early next yr (Feb-March).
If the light's prevented from reaching all sides or it's very close to something else it'll continue to grow lop-sided.