Good question. I would love to know also.

Always use a needle designed for stretch sewing ( ball point) and don't stretch the material as you sew as that will make it wavy.
There are stitches on your machine designed for stretch fabric but you can used just the straight stitch just make it a little longer.

How I do mine, I serge the edge first then turn up the hem or you can turn your hem twice without serging it. I set my sewing machine to a very slight zig zag. that will give you a stretch stitch. it will be a straight stitch with a slight jump over. play with the zig zag until you get the setting you want. don't pull your fabric or push it through just guide it. The built in stretch stitch in the sewing machines seems to stretch the fabric and make it look wavy. I've always done my stretch stitching this way long before they put it in sewing machines. Hope this helps Linda

I serge first, fold the hem and use a 4mm (stretch) twin needle. If I want it absolutely perfect I cheat: I put a small strip of tear away underneath.
Or when I have to do a few I use my Brother coverlock to stitch the hem (too lazy to get out that machine for a single hem).

I use any stitch that has a bit of stretch and I sew it with my walking foot!

Now that everyone has given you good tips, I'm going to suggest making them prettier wavy hems, what they call "lettuce". Set a zigzag stitch very close to the edge and stretch the fabric as you stitch. This is pretty on a cotton interlock neckline, t

I use the cover stitch on my serger....perfect every time...

Some great answers for me too!

Thanks everyone! I ended putting a small strip of wss on it, serging and then zigzaged. Worked great.

Just to see what would happen, I just cut the hem off one tshirt and washed the tshirt without any stitching. It made a nice rolled hem. I wore it to sheepherding and people started asking how to do it and how great it was! Oh well. I like a nice flat hem without waves.

You can use a twin needle with woolly nylon in the bobbin on a normal sewing machine .The look is the same as you would if using the cover stitch on a overlocker.Twin needles come in different widths ,just pick the one you want ,turn up the hem & sew.Remember to adjust your machine for the twin needle sewing.Do a trial first ,in case there are some adjustments to be made.