Creating a larger painting from a plein air sketch

Keith here today.
I'm writing this from the deck of the luxury cottage we rented to run our Muskoka Art Retreat.  The students have finished their three days and have gone home with their new paintings. I'm looking over the small sketches I did over the past few days and am thinking about large canvases.
Once in a while one of our small works simply calls out to be enlarged. When it does, we take off our plein air hats and move into the studio, pull out a big surface and give it a go.
I've done quite a few big versions of smaller works and have come up with a few "rules" to guide me:

1. Resist the temptation to copy the original exactly. The verve of a small painting does not translate directly onto a larger surface when you copy it exactly.
2. Don't get lost in irrelevant details. If they weren't important in the small work, they likely won't be important in the larger work.
3. Use appropriately sized brushes. When I create a 30" x 40" painting from a 12" x 16' plein air work, I use 2" and 3" house painters' brushes to keep the boldness of the original.
4. If there's something wrong in the small painting, it's okay to fix it in the bigger work. However, keep in mind rule 2.
5. Use big arm movements to apply the paint. You will not be able to emulate the spontaneity of the original by handling your brush the same way you did in the small work. You have to scale up  your gestures and your intensity. 
6. Remember, you're an artist, not a photocopy machine. Your small painting is only a reference. Make a new painting!

Here's a painting I did at The Brush for the Highlands plein air event in Haliburton in 2011, followed by a 24 by 36 I did in the studio the following winter. 
11 x 14 to 24 x 36

As you can see, I kicked up the colour, eliminated objects, added more power to the sky and tried to recreate the feeling I had looking at this slice of the Canadian landscape.
In the following pair, I wanted to take my already somewhat loose painting and take it to an almost abstract vision. 
12 x 16 to 30 x 40

The next two paintings are by Helen. She felt she cropped the original scene too tightly and lost some of the drama of the path in the woods, so from memory and her original painting, she "backed up" and created a deeper version with more vivid colours. She tells me it's a work in progress. 
12 x 16 to 30 x 40

I don't always explore new ground with a painting. In the following case, a number of people asked me to paint a larger version of the original sketch, which I did, without trying for anything new. It was accepted into the Quest Spring Juried show and subsequently sold. Similar, but different. 
12 x 16 to 16 x 20

Just for fun, I also painted a scaled down version.
12 x 16 to 8 x 10

I hope these "rules" help you in your next enlargement project. Have you enlarged any of your smaller paintings? Do you have any insights you can share?
Keith