It is halachically permissible to use hormonal contraception to delay or extend the menstrual cycle. However, the possibility of staining resulting from such use has halachic ramifications that must be taken into account. If a woman consistently stains during extended use of the pill, it would be better to avoid this usage rather than rely on multiple leniencies to prevent becoming niddah.
Many women are able to take two packs back to back without breakthrough bleeding. Other women find that if they gradually extend the active hormones by adding an additional week each cycle, they are less likely to have such breakthrough bleeding.
Some newer formulations of combination hormonal contraceptives are designed so that you get your period only four times a year, or stop menstruating altogether. In principle, this creates no halachic problems. In practice, there is anecdotal evidence that these formulations cause significant breakthough bleeding that could make you niddah. Therefore, it may be more practical to extend your cycle by combining packs of the standard 28-day pills, with your physician's approval and supervision.