Make sure that the plant is getting plenty of sunlight. Oleanders can tolerate partial shade too.
Deadhead flowers once they die out. This encourages new blooming.
Prune the tip of stems after the flowering period to stimulate new growth. Also, snip off any crisscrossed, broken, or...
Remove pests and mites from the tree. You can also use a neem oil solution to prevent any fungal or pests infections.
Grow it in well-draining soil, porous, and fertile soil.