Here's my used plastic bag.
First, flatten out you plastic bag like this.
Fold the bag in half.
Fold in half again.
Now fold the bottom right corner up to meet the edge of the bag to make a triangle.
Fold the triangle upwards as shown in the photo.
Fold the left corner up to make the triangle again. Keep folding in this manner until you reach the top.
Leave about this much, (or until you can't go any further).
Tuck the remaining part into the pocket of the triangle.
This is how it should look like when it is tucked in properly! It is very easy to keep and reuse when you want to since you can tell relatively the size of the bags by comparing the triangle sizes.