Find their interests. Experienced teachers do this by getting to know their students, through icebreakers and introductory activities, focusing on names, hobbies and occupations. Appealing to a student's interests is a main part of engaging someone of any age.
Keep the pace. Don't let the lessons get bogged down in technical details. Find your own "big-picture" approach to the subject that teaches the "why" more than the "what." This will keep your students focused; they can find the smaller details themselves as they continue learning.
Find the right activity mix. Do only what's necessary. Your adult students are already knowledgeable on a wide variety of topics; don't hit them over the head with redundant or tedious material, such as grammar in language. Teaching below an adult student's level is a common mistake and has a negative effect. Try to work with your adult students to keep ahead of them so that they are challenged.
Find "hands-on" activities. Work to apply your lessons to the everyday, letting students practice the teachings in a realistic environment. For language, this is conversation. For IT, it may mean getting students into actual engineering or programming environments. Just lecturing will not help your students like the hands-on aspect will, and as educations evolves, instructors are learning this crucial part of adapting to modern methodology in teaching.
Use group work and group activities to keep your students engaged and challenged. In any group of adult students, there is the potential for a larger interaction. Set up tasks, quizzes or projects where your students work with each other, delegating roles and solving problems in groups. This is a very effective way of enhancing your lessons.