Ok, readers, I've said it here before but this bears repeating: Test-drive your fabric before you sew your garment. You need to know—ideally prior to cutting out your pattern—how the fabric will handle all the details. We're talking about facings, hems, darts, zippers, pockets, plackets... all those things that make a garment special.
I'm in the process of making a jacket out of black leather and black rayon jersey. Sewing the leather is easy, but I swear this rayon jersey will be the death of me yet. Because it's so thick and joining two layers could be too bulky, I test-drove several finishes before I sewed the first seam. You can see my efforts in the photo above. I even tested fusible interfacing and invisible hand stitching.
Trying out neckline and hem options first on scraps showed me that I wouldn't be happy with my original design ideas. I saved fabric and sewing man-hours, and I gave myself a big pat on the back for being so smart for once.
Tell us: Do you test the details first when you sew, or do you just dive right in and hope for the best?