Add a preference for mined blocks to be the one with more transactions. This comes into play when 2 blocks of the same height are found. The first good block mined would be orphaned if it had less transactions than another. Optionally, have this rule apply to the current block and the previous one.

This increases incentive for full blocks because a miner thinking the faster propagation of a smaller block will win him the reward, but that would no longer be a good assumption.

I read some miners could attack a chain by mining small or empty blocks. This makes that a little more difficult, but they can still attack the chain many ways.