I have found that the Forerunner 205 is one of the most accurate device for tracking. It tracks accurately in situations where other devices do not (in a Gondola, in heavy canopy). It also can be worn on the arm which seems to be an ideal location for the device. If you are using a device that cannot be worn on the arm make sure you put the device where it has the best line of site to the sky. Backpacks are good and ideally you can place the device in a upper pocket. Places where your body gets between the GPS and the sky (i.e. pants pockets) are the least ideal place to put the GPS.
Accurate data is most important when you are skiing in a new area and SkiTrak(tm) is defining the lifts. If the lifts are off they can be adjusted manually in SkiTrak ™. Once you have the lifts defined accurately bad data and lost signal (short term) will have less effect on your results.
It is important to not turn on your unit until you arrive at the resort (the parking lot or when you leave the lodge is good) and to turn off the unit before you leave. This will prevent SkiTrak ™ from thinking the resort is larger then it really is. You should not start and stop the unit tracking while riding lifts and skiing, this will cause problems in generating accurate results.
If you have any questions feel free to email me.