We do individual showings. I think people are more likely to show up to an appointment than an "open house." We like to chat with people as they view the apartment so we can get a sense of whether they're likely to be good, long-term tenants. We're pretty flexible with scheduling viewings, but we live in one of the units in our triplex. I think you'd get more interest in viewings if you scheduled people at 15-minute intervals one evening and one Saturday (as sokoloff suggested, later in the day).
We also put a sign in the yard, and surprisingly, it generates the most "real" interest, but that may be due in part to the specifics of our neighborhood (older, suburban, not likely to appeal to young people). We include the # of bedrooms and price on the sign.