Have you ever sewn a seam just to find out that the bobbin ran out about 5-6 inches ago? Many of the newer sewing machines let you know (somewhat accurately) when the bobbin is low. But, here is an idea if you sew on a machine that doesn't have that new-fangled technology.:
~ Pull out about one yard of your thread.
~ On the "back side of the one yard (side towards the spool), make a mark that you can see but would wash out. There are several brands of wash-out marking tools for sewing.
~ Start the thread on the bobbin just a bit by hand and then set it up to wind on your machine.
Now when there is approximately 1 yard left of bobbin thread your mark will show up on bottom side of your seam! =D Yay! no more bottomless seams!