Often we are asked how one can continue creation of the same annotation without the user having to select it by type again.

This is easily enough accomplished by hooking into the annotationCreated event.

void 
Annotations_AnnotationCreated(object sender, EventArgs e) {
    if 
(e.Item is WpfRectangleAnnotation) {
        this.AnnotationViewer.Annotations.CreateAnnotation(new WpfRectangleAnnotation); 

    } 
    //Add each annotation type you intend to use. 
} 

When you're done creating annotations call:

this.AnnotationViewer.Annotations.ExitCreateMode();

Original Article:
Q10393 - HOWTO: Continuously draw an annotation in WPF