|I always have trouble hemming knit pants (sweats, yoga pants, etc.) in that they tend to ruffle and stretch a bit as I sew. Hem Fuser is a tape in different sizes (I use 1/2") made by H.A. Kidd and Co. I bought it at my local fabric store. You press up your hem to the length you want then put the tape between the top of the hem and the pant leg. The instructions say to put a damp cloth over top and press at cotton setting for 10-15 seconds. I didn't use the damp cloth but gave the iron a blast of steam instead. I also didn't press down for 10 seconds, more like 5-7 as I didn't want to scorch the fabric. This gives you a nice firm but flexible hem to sew and there is no stretching. I usually use the flatlock stitch on my machine which gives a nice finished look to the pants.
*Forgot to mention the price. Under $4 CDN and the roll will last a long, long time*