An effective serve is developed by continuous and disciplined practice. Even an individual with a strong serve must practice regularly to maintain the value of this primary weapon. Two baskets of balls every other day is enough to slowly improve or keep an edge on an existing serve. This amounts to 150 serves per session. Practice directional control within both serve boxes, first down the T, and then to both the forehand and backhand corner. Master these skills with relatively flat serves and then move into the realm of spin control.
Spin is an integral part of deception, control, and consistency within serving. Work on slice and topspin serves within the previously described framework of directional control.
Do these things regularly and you will develop a serve that is not only reliable, but extremely effective as an offensive weapon.
