I do several things: I eat ground flax in cereal, etc, I add flax oil to things like smoothies and hot cereals. I try to watch my intake of omega-6. I also watch my intake of saturated fats (relatively easy on a vegan diet) and limit trans fats - both of which can reduce the conversion rates to DHA.
From research I'm done, I'm somewhat confident that our bodies are capable of producing enough DHA. It's our 'unnatural' diets that interfere with the conversion (higher omega-6: omega-3 ratio, more saturated and trans fats, etc). Our diet has changed so much from the one we evolved eating that our bodies can't really handle it.