There are several factors which are important to a successful 3D/4D ultrasound. The most critical factor to obtaining a good look at your baby is how much space there is remaining in your womb. Unlike 2D ultrasound which sees through everything (this is why you can see the organs and skeletal structure) in 3D everything becomes "solid". Therefore whatever is in front of the baby, (i.e. the placenta or umbilical chord), acts as a barrier to seeing the baby's face. So the more room left, the better the chance of seeing a clear view of the baby.
The baby starts gaining fat at 24 weeks, so by 26 weeks they typically have those nice chubby cheeks everyone loves to see. Therefore it is generally ideal to have your 3D/4D ultrasound session between 26 and 28 weeks. Sessions can often be successful beyond this point, but with each week the space remaining decreases.
Hydration is Key (This can not be stressed enough) The other contributing factor to how much space remains in the womb and how clear your pictures will be (and especially for the health of your pregnancy) is how much amniotic fluid is present. So it is important to keep your self well hydrated throughout the pregnancy and drink a little extra for a few days prior to your session .
Placenta Position Another significant factor which may impact your 3D/4D ultrasound session is the position of your placenta. If you have an anterior placenta, your placenta is on the front side of your womb and in front of the baby. If you have a posterior placenta, your placenta is on the back side of the womb and in back of your baby. Neither have any health implications, so it is not frequently something your healthcare provider will tell you unless you specifically ask. A posterior placenta is ideal for 3D/4D imaging. But because the anterior placenta is in front of the baby it can block portions of the baby's face due to lack of space. So if you have an anterior placenta it is even more important to try to stick to the 26 to 28 week timeframe and maintain good fluid levels.