There are a number of ways to improve accuracy but if the project is set up well (no incorrect references) the biggest improvement will come from using a properly calibrated camera. If you are using a calibrated camera already, then you should consider the precision of the markings (for example, can you use targets that PhotoModeler can mark to sub-pixel precision), and then consider redundancy. Redundancy comes from adding more camera station views of the object with good overlap and good angles.
Please have a look at the matrix describing the factors affecting accuracy.
If you are doing a target-based industrial measurement project you might want to see more detail in this article.