This is a fun technique to expand your wardrobe.  This project is for a blouse where one side will show from beneath a jacket or sportcoat.  We've combined the techniques from Sew Fast, Faster, Fastest, by Sue Hausmann, and also Susan Khalje/Sandra Betzina's reversible tank top technique from Sew Much More, episode SMM-144, posted on HGTV's website. 

Instead of a tank top, we've used a sleeveless blouse pattern from Vogue.  The neckline on the backside of the blouse is similar to the frontside. You can choose one that suits your figure.  We've also selected four fabrics (about 1 yard each) that are easily distinguishable in these pictures (we probably wouldn't wear these), and we've also used dark thread for the sake of photography.  You should select fabrics that work well for your environment, and have similar characteristics and qualities so that they hang correctly.  You can also select two fabrics instead of four if you don't always wear a jacket, and make a simpler reversible blouse, rather than the four-sided version shown here.  We also found that this technique will work for a reversible summer dress with a similar neckline and armhole design.

Our fabrics selected for this project are 1 pink cotton floral, 1 yellow cotton floral, 1 beige floral, and one black and purple check.
Pre-wash your fabrics and press.  Stack 1 yard of each of your 4 fabrics.  Pin your pattern pieces to the fabrics.  Cut out your pattern pieces with a sharp rotary cutter or scissors.  Interface the neckline and armholes of the blouse on each piece.
Sew the shoulder seams together of fabric #1 and #2, then do the same for fabric #3 and #4.  Place the right sides together.
Pin the necklines, lining up the shoulder seams exactly.
Sew the neckline, then turn to right side, press, and topstitch if desired.
Next, we'll sew the armholes (the tricky part!).  Labeled here are RS=right side, and WS= wrong side of each fabric.  Take fabric #1 and flip over to fabric #2 with the right sides together and stitch the armhole.  Then, take fabric #3 and #4 and stitch those armhole seams together the same way.
Now, stitch up the side seams.  First, stitch fabric #2 and #3 with right sides together.  Then, do the same for fabric #1 and #4. After you stitch the side seams, you can add topstitching to the armholes if desired.
You can hem the bottom edges by either serging and leaving them loose, or folding under 1" and stitching the edges.