|I have a coverstitch machine, but sometimes it's just easier to use my twin needle.
To prevent tunneling on knit tops, I serge/zigzag clear elastic (width of elastic = width of hem, if possible) to the wrong side of the hem, then turn and topstitch with the twin needle on the right side. The elastic provides the extra "beef" needed to prevent tunneling, plus it keeps the hem from getting too wavy or stretched out. The clear elastic doesn't seem to add any visible bulk on most knits. I like this finish so much better than the Steam-a-Seam 2 or wash away stabilizer techniques -- I just can't seem to get those techniques right, and I've found the elastic to be much more stable.
I also use this technique on my gym clothes, but I'll often use poly knit elastic as it's available in wider widths than clear elastic and is a better match for the thick, sturdy knits I use in my workout tops.