Article: How existing cropland could feed billions more
The article points out a number of ways for increasing cropland productivity while at the same time reducing the environmental impact of farming.
Increasing efficiency of the farming itself, better use of irrigation water, shifting grains away from feeding cattle, increasing yield (especially in Africa).
For me, a major point is figuring out to reduce excess nutrients on farmland. While that will not increase yield directly, shifting fertilizers away from areas where they are over-used (the US and Europe) to areas where they are under-used (Africa and Asia) would.