what DJJ said
I have answered this on other forums, but short form
pray before beginning to plan your lesson, and before each class.
read your catechist guide, especially the material in front and back on the age group you are teaching, goals for this year, vocabulary, methods, various enrichment resources and activities.
get the syllabus from your DRE, and try to cover the material specified for your grade.
if the 3rd grade is teaching the Creed, 4th the commandments, and 5th the sacraments, and everybody keeps trying to teach the sacraments, obviously some kids are going to miss out on key teachings.
plan your lesson, in writing. you should spend about 45-min to an hour planning for every hour you teach (if you have been doing this for years like DJ it wont take as long.)
if you only have 15 minutes, pray with the scripture verses prescribed for the lesson, most lessons begin with or include a scripture verse (or paraphrase for kids, read the original, meditate and pray for at least 15 min.)
if you only have half an hour, next read the catechism sections given in your catechist manual for that lesson, underline key points, pray to the Holy Spirit for your own understanding of the underlying doctrine
if you have more time, read the whole lesson and plan on paper how you will
introduce the concept
teach the concept
reinforce the concept
review (next week) the concept
evaluate your lesson after the kids leave, on the back of your lesson plan.