Here are a few tips and observations from a few tough quilt blocks I am making:
Steam first, press second.*
Press. Press. Press. Press some more. When you are working with lots of corners that need to be exact make sure you pieces are starched stiff. When they are stiff there isn't much give. Unless your pieces are cut wonky to begin with, stretching won't be an issue.
Measure thrice, cut once! And measure again after cutting. It is so easy to cut 1/8 off here or there, but 8 of those 1/8 = 1 inch! Refer to above tip... press, press, press.
Cut big, trim down! Even if you follow the rule of "measure thrice, cut once" sometimes the 1/4" seam is off, no matter how much you try or how perfect you think it is lined up. Remember, you can always cut off, but you cannot put it back on.
It is okay to break the rules. This is the definition of a pattern: a model or design used as a guide in needlework and other crafts.
I prefer to work from the inside out when building the blocks. If it is a 9 patch, I start with the center square. It is my guide. Sometimes this means looking at the end of the instructions and working back. Do what works for you!
Most of all experiment to see what works the best for you. No matter how long one has quilted, they spend their day with their seam ripper. It's okay! Perfection never comes. Just enjoy the process.
*See Tips and Techniques for information on steaming, pressing and ironing.