You may be disappointed by this, but the secret of shape wear is that there is no single secret! There are actually several different fabrics used for this purpose.
Good shape wear has to do two things well – stretch/hold and breathe. Because of the latter factor, fabrics like nylon and polyester are out. Spandex can be added as a layer on top of the shape wear fabric if you want that shine, but it can’t be part of the shape wear fabric itself or you’ll feel really uncomfortable wearing it and you’ll sweat and stain your clothes. What’s more, spandex isn’t strong enough to take the strain when woven directly with lycra, so clothes made with ‘shape spandex’ don’t last long, are prone to tearing and quickly lose their shine.
The best shape wear is generally made with lycra and cotton. Look for at last ten percent lycra in the fabric you buy (there are several varieties available). This will give you the stretchiness and strength you need. Meanwhile the cotton will allow your skin to breathe and will absorb sweat, making it pleasant to wear no matter what you’re doing.
To sew shape wear with spandex, stretch out both the shape wear fabric and the spandex as tight as they’ll go (without damaging the sheen on the latter) and fix them together, keeping them taut as you sew. Use a fine needle and a hemming stitch, which will distribute the strain through the fabric in several directions, reducing the risk of seams tearing. Take a tip: use fabric tape for fixing, rather than pins, which can leave unsightly holes. Run the tape down the inside of the fabric so as not to damage its sheen.
Shape wear is best constructed using panels, so that pressure in one area doesn’t create strain all round he body. It works well in smaller garments such as pieces of lingerie, but less well in complete outfits – in the latter case, consider wearing shape wear underneath with something else on top. Alternatively you can use a nylon-lycra blend for a stretchy garment that will offer a degree of support, but this will have a limited life span, especially if you’re active whilst wearing it, and it won’t retain a shiny look for long.