If you had enough interest to read my "About" page you already know that I have been sewing since I was very young. My mom sewed, so it was natural for me to take an interest in it as well. However, I should also tell you that I stopped sewing for a quite a while too. The reason was primarily time related but I do remember having complete frustration with fitting issues and not knowing how to correct them. Many projects became unfinished, bagged, hidden and eventually thrown out. Surprisingly, a four year degree in Fashion Design didn't teach me much about fitting. I acquired my current skills on the job, with practice, and by referring to books on the subject. To me this means that if you are motivated, anyone can perfect their fitting skills. I hope to pass along what I have learned and I welcome your input as well.
Plan Your Approach
As many of you already know, it is really difficult to perform a fitting on yourself so it is a good idea to enlist a friend to help or strategically place some full length mirrors so that you have full view of yourself front and back without turning your head or torso. This is important because any movement or posture change will be seen in your garment in the form or "wrinkles" or "drag lines". The use of a dress form is very beneficial if you have one. It's always easier and less confusing to assess fitting corrections if your muslin is a little big on you so cut the larger size if your measurements are not matching the pattern exactly. Be sure to transfer center, bust, waist, and hip lines to to the test garment.
Assess the Fit
Stand straight in front of the mirror and check to ensure that the center front and center back of the garment is lined up with the center front and center back of your body. Look for conspicuous drag lines or wrinkles. In a good fitting garment horizontal seams such as empress, waist, and hemline are level with the floor and there are virtually no wrinkles when your posture is static. If you see any of these issues in your garment, your sole purpose is to eliminate them. Review your garment from top to bottom moving from front to back as you progress down to the garment hem.
Make Length Adjustments First
Length adjustments ensure that the position of the contoured areas such as armhole base, bust curve, waist position, and hip curve are sitting at the same level on the pattern as they are on your body. In general, you will see soft horizontal wrinkles pooling in areas where lengths are to long, and vertical tension (pulling) wrinkles in areas where lengths are too short.
Make Circumference Adjustments Second
Circumference adjustments ensure that you have enough ease for normal movement without being overly roomy. In general, you will see soft vertical wrinkles in areas where the circumference it too large and horizontal tension (pulling) wrinkles where circumference is too small. Examples of circumference adjustments are shoulder width, chest girth, waist girth, and hip girth.
Making your pattern adjustments in this order first puts the contour areas in the correct position and then allows you to fit those contour areas to your specific body size and shape.
I have two really great resources for fitting information: Fitting & Pattern Alteration-A Multi-Method Approach and Dress Fitting-Classic Edition. Neither of them are easy reads but with some patience, they hold a lot of valuable information. Fitting skills are primarily developed through practice and experimentation but a little guidance doesn't hurt either. If you've got some good fitting resources, add them to the comments.