Take the pattern piece you would like to lengthen and draw a horizontal line through the centre(ish) of the pattern. Make sure the line is perpendicular to the grainline. This will be your "cut line" which can be used for both shortening and lengthening your pattern.
Take your scissors and cut along the line so that your pattern becomes two separate pattern pieces.
Decide how much you would like to lengthen the pattern by. Cut a piece of pattern paper approximately the width of your pattern and the length you'd like to add (with a few centimetres either side).
Draw a straight line across the piece of paper. Measure down from this line, the distance you are lengthening the pattern by and draw a second line at this point (parallel to the first). For example, if I want to lengthen a pattern by 12cm, I will measure down 12cm from the first line and then draw a second horizontal line.
Take your two pattern pieces, and, with masking tape, tape the bottom of the top pattern piece to the first line, and, the the top of the lower pattern piece to the second line.
Once the two pieces are securely in place, redraw your seams with a ruler (in this case the centre front and the side seam), from top to bottom.
Trace off your new pattern piece - and there you have it - a longer pattern!