Here are two resources to help pastors maintain their Greek skills and use them effectively in sermon preparation.  

Black, David Alan.  Using New Testament Greek in Ministry: A Practical Guide for Students and Pastors.  Grand Rapids: Baker Book House, 1993.  This book focuses on the transformation of exegesis into exposition and of studies into sermons.  It explains the process of using the Greek text and linguistic resources to study the New Testament.

Campbell, Constantine R.  Keep Your Greek: Strategies for Busy People.  Grand Rapids: Zondervan, 2010.  Seminarians spend countless hours mastering biblical languages and learning how the knowledge of them illuminates the reading, understanding, and application of Scripture.  But while excellent language acquisition resources abound, few really teach students how to maintain their use of Greek for the long term.  Consequently, pastors and other former Greek students find that under the pressures of work, ministry, preaching, and life, their hard-earned Greek skills begin to disappear.  Con Campbell has been counseling one-time Greek students for years, teaching them how to keep their language facility for the benefit of those to whom they minister and teach.  He shows how following the right principles makes it possible for many to retain--and in some cases regain--their Greek language skills.

      -- From David Brewer, Annotated Select Bibliography

Though I prefer Greek Behind the Prof's Back ( of course!), it seems only fair to provide information on some of the other textbooks available for beginners in New Testament Greek.  You can always use the old classics by men like J. Gresham Machen or Ray Summers.  They have been standard fare for decades, and you can still use them to get started in Greek.

However, there are several more recent texts that have come on the market in the last few years and I offer these brief descriptions (taken from the Annotated Bibliography by David Brewer) to help you evaluate them:

Black, David Alan.  Learn to Read New Testament Greek, 3rd edition.  Nashville: B&H Academic, 2009.  This user-friendly introduction to New Testament Greek keeps discussion of grammar as non-technical as possible.  It provides tools and exercises for bringing the student to the experience of reading from the Greek New Testament after just seventeen lessons.  Among the revisions to this third edition are updated discussions and scholarship, and additional appendices.

____________.  Learn to Read New Testament Greek - Workbook: Supplemental Exercises for Greek Grammar Students.  Nashville: B&H Academic, 2009.  Keyed to David Alan Black's popular Learn to Read New Testament Greek main text, this supplemental workbook includes 1300 Greek to English / English to Greek sentences, more than 700 drilling exercises to reinforce the foundational principles of Greek grammar, and many other helpful learning resources for introductory Greek students.

Croy, N. Clayton.  A Primer of Biblical Greek.  Grand Rapids: Eerdmans, 2007.  Taking a primarily deductive approach to teaching Biblical Greek, this volume assumes that students will have no prior knowledge in Greek. Divided into 32 separate lessons, each containing a generous number of exercises, the text leads students from the Greek alphabet to a working understanding of the language of the Bible.  The new accompanying CD contains a teacher's guide, answers to exercises, and more exercises.

Hewett, James Allen, C. Michael Robbins, and Steven R. Johnson.  New Testament Greek: A Beginning and Intermediate Grammar.  Peabody, MA: Hendrickson, 2009.  For over twenty years, first-year Greek students have relied on James Allen Hewett's New Testament Greek: A Beginning and Intermediate Grammar for its straightforward approach to the fundamentals of language study.  Now completely revised and updated, this trusted grammar will provide a new generation of beginning scholars with a solid foundation for doing translation, exegesis, and biblical interpretation.

Mounce, William D.  Basics of Biblical Greek Grammar, 3rd edition.  Grand Rapids: Zondervan, 2009.  First published in 1993, Basics of Biblical Greek is the most popular introduction to the field, used in universities and seminaries around the world.  Over 200,000 students have learned biblical Greek under its guidance.  This significant third edition has been carefully developed in consultation with instructors, students, self-learners, and homeschoolers.

_____________.  Basics of Biblical Greek Workbook, 3rd edition.  Grand Rapids: Zondervan, 2009.  Expanded, student-friendly workbook for use with Basics of Biblical Greek textbook.  This expanded workbook is designed with the student in mind.  Two optional chapters have been added, allowing you to read large chunks of the biblical text and enjoy the fruits of your labor faster than every before.  Each chapter is divided into six sections and includes extensive exercises and significant biblical passages for translation.

_____________.  Biblical Greek: A Compact Guide.  Grand Rapids: Zondervan, 2011.  Offer Greek students a one-stop guide for the grammar, morphology, and vocabulary of biblical Greek.  This resource follows the organization and format familiar to the hundreds of thousands of students who have used Basics of Biblical Greek Grammar in their first-year Greek courses, but it is also usable by students who learned with a different grammar.  By limiting its discussion to the "nuts and bolts," it allows Greek students to find the relevant grammatical refreshers more quickly and easily when they are working on translation and exegesis.