Read this tip to make your life smarter, better, faster and wiser. LifeTips is the place to go when you need to know about Finishing a Project and other Crochet topics.
The shoulder seams are probably the most important ones of your crocheted sweater or jacket -- they carry the weight of the entire garment. So, it's important to make these seams really strong, using yarn or thread that won't break or distort the seam. The backstitch is probably the best way to secure the shoulder seams --it's well-suited for creating a straight seam on the right side of the garment, especially across any increases or decreases that were used to create the shoulder shaping.
The backstitch involves a "two-step forward, one-step back" technique. Think of a "T" -- if you are right-handed, insert the needle at the right end of the top of the T and bring it out at the left end, pull out the thread and insert the needle again at the middle of the top of the T and bring it out in the middle of another T as if it were to the left and next to the current T. Then, insert the needle where you started and bring it out at the left end of the top of the second T. Keep repeating, working your way from right to left (reverse these directions if you are left-handed).