They quickly spoil. Rinse produce thoroughly before juicing. Scrub if necessary with a natural bristle brush before juicing. Cut produce into small pieces just before juicing. Don't cut it and store for any amount of time.
Strawberries especially lose their freshness very rapidly once they are cut.
Also leave any stalks attached until just before juicing. Leave the skin on just about everything. Exceptions would be oranges and grapefruits. You do not generally need to remove seeds. Do remove pits from fruits such as peaches or apricots.