For the increasing number of RME users, a compilation of possible measuring errors is made. A detailed description of sample preparation is given, starting from granular as well as powdery polymers. The preparation of smooth surface sample for optical rheometry is also addressed. A large part of the paper deals with the evaluation of true strain rates by particle tracking. A discussion of the errors arising from using different strain rate measures is made. Data on the elongation of LDPE at <() over dot> = 0.01 s(-1) is given to show the possibility of increasing the measuring sensitivity by repetition and averaging. The surface tension of the LDPE melt is measured as an example. The influences of sample thickness and the diameter of the distance pins on the measured viscosity are shown. The influences are small, and an upper limit of 1.5 mm for the sample thickness is recommended. The possibility of performing stress relaxation and recovery measurements after cessation of steady elongational flow is mentioned but not supported by our own measurements. The same holds for optical rheometry.