For open notes only, you can use your LH to dampen the strings at the nut. Pluck with your right as usual.
Another technique (which was customary in the 19th century) is to play with your fingertips exactly on the frets—that way the flesh of your finger, extending beyond the fret, will dampen the string lightly to produce a muffled tone. Alternatively, you can use a second finger to dampen immediately in front of the fret.
Finally, you can play pizzicato by using an extremely light touch and releasing the pressure as soon as you sound the note. This to me is the most tricky way to perform the technique, but it can be developed to play some pretty fast pizzicato passages that would be impractical otherwise.