Quick google says your Sony a550 can do back-button AF, here
http://forums.dpreview.com/forums/post/34282416
Suggest use centre AF point, and focus-recompose technique. Checking and rechecking focus is very important as there's often less depth of field than you might think, especially with head and shoulders when subjects are moving a bit, like leaning backwards and forwards etc, unless you're at a high f/number.
You need to know that focus-recompose can lead to inherant errors in some situations, but only when a) you're very close, b) the subject is positioned a long way off-centre, and c) you're using a very low f/number. It's an unusual combination of factors and unlikely to be a problem but just be aware.