I've always have had problems in the past with all of the things that I have made from store bought patterns.
That's because I have been measuring myself wrong. After a little research, I found the proper way to to it. I measured myself a few times to make sure that I was doing it right, and each time I came out with the same numbers. And after using my proper measurements I actually made something from a pattern that turned out to fit me correctly.
Here's a quick guide, it's pretty simple.
Bust: Around the fullest part of the bust. Around your back and up under your armpit. (with your arms down, you should be able to hold up the tape)
Waist: At your natural waist line. (It is higher up then you think)
Hips: The outer most part of your hips.
Inseam: The distance between the inner most part of the thigh to the bottom of your ankle.
Sleeve length: Start at the middle of the back of your neck and measure from your shoulder, then to your elbow and then to your wrist.
If you have difficulty with any of these a partner is a good way to help you get my proper measurements.