I don't know if this would work for other people, but forcing someone to use metrics for a critical part of their life, even for only a few weeks, can help metrics make sense. I've generally been in favor of a metric conversion, but never had a serious opportunity to make that jump until last year.
When my son was born premature, he was in the hospital for seven weeks. Just about everything we did was metric: his weight, his meds, the amount of milk he took was in various metric measurements. About the only non-metric measurements were his temperature and length. He was only in the NICU for a week before being moved to Pediatrics, and we basically lived there with him for six weeks, participating in most of the activities associated with building up his feeding ability and core strength so he could go home.
This has resulted in us keeping many things metric for him because it makes sense to us. We still weigh him in kilos (and have to convert that to pounds for others). While he was still on milk, we measured that out in milliliters, and when we mix his formula, everything is in grams (570g of water, 145g of formula powder, and 28g of corn cereal combined for four bottles of ~175mL of formula). Talking to his pediatrician, we have to convert to ounces (we just call it "about six"), and we have to convert recommendations from tablespoons and ounces to metric equivalents so the scale seems right to us.
We still haven't quite latched onto distance measurements, but that's probably just be a matter of finding the right scenario and using it almost exclusively.