What is a Response Journal you ask?  It’s basically a journal of responses to something you are reading.  This site explains it as:

Keeping a response journal will give you an opportunity to express your own opinions about what is happening in the novel you are reading. Passages that upset you, make you happy, or that you simply do not understand, can be discussed in your journal entries.

The site also has a great list of prompts to use.  A similar resource, titled "What is a response journal?", is available at Instructional Strategies Online.  This site includes a number of PDF templates to download and print.

Busy Teacher’s Cafe has a great bookmark template of simple response journal prompts.  You can view the PDF template here.  Print one out and use it as a bookmark in the next book you read.  You can jot down notes on it as you read.

Some more prompts and some tips on writing a response journal can be found in the Lesson Bank at Teachers.net .  There’s also a very nice list at brtom.org, these ones are more for older kids and adults - they are well written and will really get you thinking.