Another very handy trick from Andy’s kitchen. When you do mirror modelling, at some point you end up merging the two instances. But then you realise you need to add more detail. What I did till now was to nuke the other half and create the mirrored instance again.
However it is usually easier to mirror only the newly added detail. Just snap the 3D cursor to the appropriate center, duplacate the new shape, mirror, remove doubles. Voil??.