Enlarging garments takes some creative thinking. If the seam allowances don't give you enough fabric, you'll have to find it somewhere else. Rather than making your alterations disappear, try to make them look like original trim and hide them in plain sight. This works best with casual wear, but some dressier clothes might lend themselves to this type of altering, too.
Pants and Skirts
Take out the stitching in the side seams. It may be necessary to cut through the waistband. Take out a little of the hemstitches on each side of the side seams. Press the old allowance flat, and trim the edges so they are even. Cut two strips of fabric as near the same weight as the garment. Cut them as wide as the amount you want to add plus seam allowances. Be sure the strips are long enough to hem at the bottom, and turn under for the width of the waistband at the top.
Sew these strips into place at the sides of your garment. Redo the hem and turn under, and topstitch at the waist.
Cover the side panels with colorful braid or woven ribbon. You may be surprised at what you can find that will go with the color and style of your pants or skirt. Adding a little of the same trim to the edge of the pockets will further encourage the idea that the trim is original. For skirts, you might want to add three off-center stripes to the front rather than at the side seams.
While these panels work well with jeans, they are really best for shorts, skirts, and children's clothing. Unless you are fixing expensive jeans, the ribbon might turn out to be more expensive than replacing the pants.
Shirts and Blouses
To use panels to widen shirts and blouses, choose a contrasting fabric to create vertical stripes down the front. Open up the shoulder seams. Cut vertical lines straight with the grain from hem to shoulder front and back and on both sides. You can sew your panels in with two seams or by folding under the edges and topstitching as you overlap it with the cut edge of the shirt. This latter alternative is more likely to give the appearance of a stripe of trim.
If there are darts in line with where you want to cut, take out the stitches up to a couple of inches from the side seams. Press the darts flat. Make the cuts, and sew in the panels with two seams. Trim the top of the panels to align them with the shoulder seams, and repair these seams. Repair the hem.
If you had to take out a dart, try the shirt on inside out and find the lines for the new darts. Be sure both sides look the same, especially where they cross the panels. Consider replacing sleeve bands with your panel fabric or adding a breast pocket.
If you only need to add a little to a shirt, add a new button panel. Whether this works or not will depend on what the present button side looks like, since it will now show. Keep your addition narrow enough to remain behind the buttonhole overlap.
If a sleeve is too tight, you can put a similar panel along the top of the sleeve. Add a matching panel across the shoulder seams or along the bottom of the shirt.
If the armhole is tight too, open the shoulder seam and widen it with a panel. You can turn it under at the neckline or redo the facing. If there is a collar, you'll need to completely redo the neckline.
Dresses without waistlines might be altered with panels the same way as shirts or blouses. If there is a waistline, you might open up the waist seam and sew in separate panels for the bodice and the skirt, but be sure to have them meet.
If the dress is gathered, you might take the skirt off entirely, or at least up to the zipper. Add the panels to the bodice, and regather the skirt onto the larger waistline. If you want the skirt to be fuller, don't remove the skirt except right at the point where you add the panels. Add slightly wider panels to the skirt so it gathers appropriately.