Hi Diane,
This is always a tough one. Even after 20 years of quilting, my stitches don't always catch on the back, so you won't see me posting pictures of the back of my quilts!
When I go straight down, I feel the needle coming through with my bottom finger. Then I rotate the needle back, without pushing with the thimble. I do press down with my thumb and up with my bottom finger - it's almost like folding the sandwich onto the needle. Sometimes, I can tell when I'm not catching the back...that happens most when my sandwich is a bit tight.
Another tip is that it is much harder to quilt on the straight of grain. If you go on the bias, there is more flex in the sandwich and you can make nicer looking stitches. You can also hide a multitude of sins by using a busy print as the backing - I do that with my machine quilting.
Janet