Optimizing data acquisition technique is necessary, in order to gather as much information as possible in a limited time, without loss of data quality.
Simplest approach to decrease the total measurement time (simultaneous measurements on multiple potential electrodes) is to sort the ERT sequence by number of current electrode. However, order in which the measurements are performed also often affects the data quality.
Electrode charge-up effects is a well-known phenomenon in induced polarization. At first, electrode is charging up when transmitting current. Then, after current injection, magnitude of the polarization is decreasing in exponential-like pattern. These effects, which occur when an electrode is used for transmitting current, should be taken into account if the same electrode is subsequently used for measuring potentials. Particularly, this has been covered by Torleif Dahlin:
Dahlin T., 2000. Short note on electrode charge-up effects three times. in DC resistivity data acquisition using multi-electrode arrays. Geophysical rospecting, 48, 181-187.
It is important to optimize the measurement sequence so as to avoid making a potential measurement with an electrode which has recently been used for transmitting current in the ground. The time-lag between times when one electrode is used for current injection and voltage measurement has to be increased. For example, for reversed pole-dipole array (MNB) it is recommended to sort sequence by current electrode in descenting order. First, one particular electrode is used in potential measurements and only after in current injection.
Generally, sorting forward and reversed pole-dipole sequence (AMN+MNB) by current electrode in ascending order leads to lower data quality for reversed array (MNB). 
Установка MNB
Figure clearly shows low data quality for reversed pole-dipole array (right). There are not visible problem for forward array for the same setup (Near Surface 2006).
For dipole-dipole and Schlumberger arrays data quality can be improved by separating in time the measurements that performed with different lengths of current dipoles. So the sequence should be sorted not only by current electrode, but by current dipole length also in ascending order. Thus, all measurements with smallest current dipole length is conducted, then for longer lengths, etc.
It is extremely important for ERT devices which allow any combination of potential electrodes (Terrameter LS, Omega-48, Syber-64). Here, same electrode can be involved in many simultaneous measurements (with regard to many available receiving channels). In case of high contact resistances it often leads to poor data quality.

Sequence optimization

First, general measurement array has to be divided into separate subarrays of the same type (pole-dipole, dipole-dipole, Schlumberger, etc.). Sequence optimization is done for each subarray separately. Thereafter, all subarrays can be merged into one general optimized array. x2ipi user can merge subarrays by Menu - Save - Merge SEQ - Merge TXT. Also any sequence can be sorted by Menu - Save - Sorting - Syscal Pro.