Photo 1
At the walk, the horse should be energetic and covering much ground.   The hind legs should come up well underneath the body.  The walk should have a distinct four-beat and the horse should be relaxed and straight.  Here Matrix (by Avignon II, out of Mallorka by Gunnar) is relaxed and straight but not energetic enough.  The slight slack in the reins shows that the handler is not restricting the horse's head.

Photo 2

When walking or trotting toward or away from the judge, make sure that the horse, not you, is lined up with the judge.  Also make sure that the horse's body is tracking straight (including the head and neck).  Matrix is good but at a very slight angle.  The back feet should follow the same path as the front feet.  The hind legs should almost disappear behind the front legs.