It's funny, because this is something I used to do in my classroom when I taught "up north" and it worked really well. For some reason I have gotten away from it but I thought it would be valuable to share anyway.
The basic procedure is to have the kids brainstorm nouns, verbs, adjectives, etc that relate to a theme or story you are working on. You can list the ideas in a chart OR I like to make sentence strips of different colors for each sentence "part" for the kids to manipulate and make into one long "juicy" sentence. They have to use one strip of each color for their sentence (1 adjective, 1 noun/subject, 1 verb, etc).
Then, I ask the kids to draw a picture to go with their writing. The picture has to show all the things that are going on in their sentence. One idea I had was to use these inexpensive writing journals from Starfall.com for this purpose. They have space for a drawing at the top of the page and space to write at the bottom. Saves on copy paper and keeps all their writing together!
Here are some generic sentence parts to get you started. Here are a few coloring conventions I use in my classroom - these are based onKathy Robinson's writing instruction. I print the sentence parts on the corresponding colors.
Here's a book that will give you some good theme ideas and methods for teaching this type of sentence building:
For even lower level students, I recommend this book for sentence building materials:
This book contains cards that can be copied (I use the colors mentioned above) so that students can build sentences using three parts: Who, Did What, and Where. I then ask them to draw a picture of their sentence. Again, the Starfall journals mentioned above would be great for this!