One method of teaching vocabulary that is particularly useful for teaching idiomatic expressions is the use of word association diagrams. For example, on a blackboard, you can draw a circle and write a word such as "easy" inside it. Then, brainstorm with your class to draw bubbles surrounding this circle to include synonyms of the word "easy" and also idiomatic expressions that mean that something is easy to do. Some examples would be: easy as pie, painless, effortless, elementary and simple. Students should copy this diagram in notebooks.
Students learn through repetition. After the brainstorming session on the board, have students take quizzes to help with memorization. The quiz should have at least two different tasks: you should include a partially filled-out diagram that students have to fill in and also several sentences that the students have to complete based on the words in the diagram. This will help not only with memorization but also making sure both ESL and native English students are comfortable using the expressions and new vocabulary in context.
In addition to student-generated lists, expose the students to as much authentic language as possible, both spoken and written. People learn through context, and augmenting the rote memorization in class with listening to radio broadcasts, watching movies and reading newspaper or magazine articles will expose students to a greater variety of expressions and phrases and will also help students get clued into their meaning by using context clues.
Native English speakers have an advantage in learning idiomatic expressions because for the most part, the words used to make up the expressions are ones they are familiar with. For ESL students, both the words in the expression and the expression itself may be new. Begin by making sure that the individual words in the expressions are clear, and promote the use of a vocabulary journal in your class so that students can keep new words handy for review. Then, use diagrams, review tasks and listening to and reading authentic language to learn and memorize idiomatic expressions.