Doing them with your hands elevated, as Dean suggests, will definitely make it easier to train the skill because your legs have more room above the floor.
If you are having trouble with keeping your legs extended straight behind you, working on planks and leg raises more will help build the abdominal strength, and working on bridges will help build the mobility in the lower back.
You can also try elbow lever back extensions. Although the full movement is more difficult than a standard elbow lever, you can do a partial movement to help build the strength up. I find the exercise most comfortable to practice on a countertop, as it gives ample room for your legs to hang vertically.