Some things I've learned along the way -
1) Make sure that all the units that one intends to join are the intended size.
2) If I have to ease something in - I put tick marks on both pieces at the 1/2, 1/4, and 1/8 intervals - depending on how long the intervals are and how much I need to ease in so that the extra is distributed evenly. Then I match the tick marks and pin. And if I am lucky and the fabrics are cooperative, I can steam/shrink the "extra" out after the seam is sewn.
3) Some fabrics refuse to be eased - batiks, for example. Generally speaking, the crosswise grain is "stretchier" than the lengthwise grain.