FX sensor are bigger so there is a higher chance of dust landing on the sensor. It doesn't mean that with the lens attached, dust will never get in. Dust get into lens, dust can get out of lens into your sensor.
With D700, you can point your body down, and use the sensor cleaning mechanism in your body. It's not perfect but it does get the loose dust off easily.
If it doesn't, lock the mirror up for cleaning, point your body down and point your blower up to blow dust off. Just make sure the tip of the blower never touch the sensor.
Dust are really not that terrible. Oil is worse. If these two steps are not enough to clean your sensor, then pay NSC to do it for you.
I went during office hours, no queue, so the wait is about 30min. A very clean job though. I had to clean it at NSC cos there were 4 specks of dust (or very mini hair) stuck on the sensor that my blower can't blow off. I could see the dust specks via liveview from f11 on. I think once every quarterly or half yearly sounds reasonable.