Usually an attack of Sinusitis occurs when mucus gets accumulated in the sinus cavities and the mucus does not come out. Since mucus is a breeding ground for bacteria and other micro-organisms, this leads to infections. Also the accumulated mucus rubs against the walls of the sinus tissues are causes discomfort.
Given this background, it is important that the sinuses are flushed regularly. As one of the participants has suggested, you can use a Neti-Pot or a nasal spray. I personally use Sinox Nasal Spray which is a non-systemic and drug free nasal spray. The antiseptics and surfactants in it rinse away thick or persistent mucus. It also has preventive / prophylactic properties which reduces the probability of suffering a Sinusitis attack.
You may also want to take up Yoga which specifically has exercises / asanas like Namaskara, Sarvangasana, Ardha Chandrasana, which can help with Sinusitis and general respiratory health.
Hope this helps.