I find it rather instinctive to "lock onto an altitude", without the need to actually read any numbers.
Just set the desired altitude in the FCU (MCP), and then keep the altitude symbol/bug (the big blue square) aligned together with the altitude indication box (yellow box).
If the blue square (altitude bug) rises above the yellow square (altitude indication), it means you are descending, so just pitch upwards.
If the blue square (altitude bug) moves downwards, it means you are climbing, so pitch downwards.
Last edited by John Citizen; 27th May 2012 at 11:17.