My first sight of oleanders (Nerium oleander) growing in their natural habitat was in the desert of Dubai. They grew in profusion on the edge of the wadis (river beds that change into torrents after heavy rainfall). Surviving for long periods without rain, makes them ideal shrubs for our climate. However, if you do water the oleander regularly until midsummer, they will flower better.
Most reference books will say to prune after flowering in early autumn, but if you live in an area like we do, where during the winter the oleanders will get some frost damage, it is better to prune in spring when there is no more threat of frost.
In general oleanders need to be pruned lightly no more than 1/3 of each branch. Branches that are damaged, weak or crossing need to be taken out. Prune the branch just before a leaf node.
If you let these new branches grow on a bit and then prune them again, each one will again split into three.
By this method you'll increase the number of branches and you have created a very full rounded shrub.
Oleanders flower on new wood. One thing to remember is that every part of the plant is poisonous even when you are burning the cuttings do not inhale the fumes !
If you’ve got an oleander that has totally grown out of control, you can cut it down to 30 cm above the ground. It will recuperate, but it takes about two years before it flowers again.
[Bibliography : International Oleander Society; eHow – How to Prune Oleanders; Plantes Méditerranéennes by Serge Schall, Mediterranean Gardener by Hugo Latymer]