My tip is one I learned by default! Always buy a little bit more fabric than your pattern says! Why, Murphy's Law: What can go wrong, will go wrong. and it's corollary: If the fabric is no longer available, you will make at least one cutting error and need extra fabric!
When I sign up for a BOM, I buy an oops kit , if offered or just buy fat eighths or fat quarters of the fabrics needed. I have rotary cut too often when I should be sleeping and cut wrong! If I am lucky enough to find the fabric, it costs me more in shipping than if I just bought a little bit extra to start with. as for the extra, it gets used in scrappy quilts!
Good tip Judi