Lecturing (teaching methods) for a diverse group of students with weak science backgrounds I just started teaching three courses at small medical training college: anatomy and physiology, medical terminology, and microbiology. Most of the students have a weak background in the sciences, and come from varying professional/educational backgrounds. 

I currently end up preparing 4-5 hours for each hour that I end up lecturing. But find it difficult to keep the students engaged for the duration of the lecture. Any advice on improving teaching/ lecturing methods?
Teaching 'unprepared' students is always a challenge and prep takes a lot of time (four to five hours of prep for one hour of lecturing sounds about right...and maybe a bit low). So, overall, your numbers seem to be inline with norms.

If you want to make your teaching more effective, you should read some books about teaching (there are many). You should read this question for some ideas on improving yourself as a lecturer.

Generally, 'active learning' - where the student is the focus of the learning process (as opposed to the lecturer being the center) - focuses on engaging students, putting the load on their shoulders. Group work is particularly useful. I've seen studies (not handy) that say that in every learning situation, group learning (e.g., group discussion) always improves the learning effectiveness.

You should do more prep and you should do less in class (they should do the work in class) and they should do work outside of class. All of this needs your careful planning but some students will think they you are not doing anything...but they will end up learning more in the end.