How can I avoid getting marks on my nipples while pumping?

See answer. The following link has some good advice form a lactation consultant... http://www.babycenter.com/404_how-do-i-prevent-blister-like-marks-on-my-nipples-when-pumpi_8882.bc.
Less suction. Vigorous pumping can lead to suction bruises. If your pump has a suction strength adjustment, lessen the suction to see if it resolves the problem. Regardless, suction bruises may be unsightly, but they are harmless.