I use painter's tape for straight line hand quilting-- just press it where I want it, quilt a line, move the tape. I especially love it for crosshatching.
I use paintbrushes to clean the bobbin case area on the sewing machine and the longarm. The hairs don't come off the paintbrushes as easily as they do from those brushes meant to do that cleaning, and wow, paintbrushes are much cheaper too.
I use old rx bottles to hold filled bobbins of thread. I hate interrupting my sewing to wind bobbins so I wind a lot of bobbins with beige or grey and put all one color in a bottle. Often one "bottle of thread" is enough to do an entire quilt.
I have long yardage of fabric hanging in the closet on pants hangers. You can put a LOT of yardage on those hangers that are meant for 5 pair of pants! I bet I have 20-30 yds on each hanger.
I have pantographs for longarm quilting stored in over the door shoebags.
I'm always dropping pins and I'm not as young as I used to be, so it's a real trial getting down to get them under the table or the longarm and getting back up again. I took one of those magnetic business cards that some local realtor gave me, glued it to the end of my yardstick, and voila, a nice long magnet for picking those pesky stray pins up off the floor.
I have a white vinyl trellis, 4 ft x 8 ft, mounted on the wall in my workroom, sideways under the window. It has S-hooks on it, and it holds dozens of rulers and stencils in a space that would be wasted. And it looks pretty cool too.