1) The fabric you choose is important.
A bag may not be durable enough if you don't use heavy-weight canvas or leather. A garment may not drape nicely if you use too stiff a fabric. A waistband might stretch out if it doesn't have enough stretch recovery. A summer top in synthetic fabric may be uncomfortably hot because it doesn't breathe.
2) Use the right needle for the job.
They may all look the same, but they don't work the same. A leather needle is essential for leather or you may end up with skipped stitches. Knits require a ball point, or you may slice through your fabrics. Use a leather or jeans needle for heavy duty fabrics or you may break some.
3) If you are making your own patterns, always add more ease than you think is necessary.
Just in case of mistakes that need adjusting. Also, there's stretch that you need to compensate for.
Don't be like me. (I've done it all wrong).