1. Find someone who has expert knowledge and is competent in all aspects of sewing, not just hems and simple quick fix solutions.
2. Find clothes from within your own wardrobe that can be brought up to date by doing the hem, taking it in or re-shaping the shoulders. Subtle alterations can make clothes look more "fashionable".
3. Don't personally do complicated alterations unless you're confident in your skills. However even if you try and can't finish the work, hand it over to a professional.
4. Before you alter new cotton clothing or a pair of jeans, wash or dry-clean them first according to the label to allow for shrinkage.
5. When you hem, pin both legs as one leg can be shorter than the other. Get someone else to pin your clothes.
6. Look into getting something tailor made for your size and shape. It may not be as expensive as you think.
For those of us who ARE plus-size, the advice in point 6 is really wise. Too often we spend time and energy in searching for something that we have a good idea on what we really want, only to take on second best. And even then it may need alterations.
Therefore if you have nice fabric and even a pattern (not necessarily in your size), take it along and have a chat with someone who you've seen or heard recommended. Talk to them about what you'd like as far as a tailored garment to suit YOUR body and shape.
We know it sometimes takes a while to find someone who "understands" the more curvaceous figure, but a really good dressmaker is worth more than her weight in gold.