OK, I pulled from LCMS sources:
http://www.ctsfw.edu/library/files/pb/896
http://www.ctsfw.edu/academics/faculty/pless/pastoral-formation.htm

And from the mouth of Luther, himself:
http://www.ourredeemerlcms.org/pdfs/oratio%20etc.pdf

Some great stuff here.

Oratio (prayer) is grounded in the Word of the Lord.
Meditatio is the continual study of the Scriptures.
God uses tentatio (spiritual affliction, trial, and temptation) to drive
us away from self and to His promises alone.

Blessings,