Mission Impossible (well almost).

[Anna]

You could also try starching the fabric, it will help keep it from stretching while you sew and have the added benefit of keeping the edges from curling. I've also had luck using woolly nylon thread in my serger (overlocker) to produce a stretchier seam, the thread itself can stretch, it also has a "fuzzy" texture so it is a bit more comfortable next to your skin.

[crazy1503]

Knits are the devil. I sew a LOT of childrens clothes with them and the best way Ive found to deal with them is to use a walking foot on your machine - it will stop the stretching and make it almost as easy to sew as cotton. Stay tape on trickey edges helps if there is still a bit of stretching

[hutch--]

I usually do OK with stretch knits on an overlocker by adjusting the stitch width and the amount of differential feed but this Lycra is so fine it misbehaves badly on any of my overlockers. I can get one to join it cleanly but the seam does not have enough stretch so I had to look for another option. I found that using a sewing machine (normal lock stitch) did not produce any stretching so I tested through the cams on my old Elna and found a pattern that does a sideways zig zag back and forth which give plenty of stretch and not stretching. Both peterle and myself use an old Elna machine that accepts a wide range of cams which allows you to do unusual things when necessary.

The real problem with this Lycra is it is so light that you have to learn a new way to handle it so that you can cut and sew it without stretching it.

[crazy1503]

Youre brave! I find sewing stretchy fabrics to be tourettes-inducing.

[Claire Shaeffer]

Greger suggested using paper. There is a wash-away stabilizer that machine embroiders use. A sewing machine dealer will have several brands and perhaps some other suggestions.
Claire