I practice on fish.

For non-fish practice, I'd recommend an actual bowfishing arrow with the barb removed as Mark suggested. You want it to fly and drop like the real thing. A sand pit is a great backstop.

Unfortunately, the hardest part of bowfishing is staying in touch with the whole "aim-low" concept, and that can only be done in the water, so anything that is actually submerged will be better at honing the most critical skill than mere shooting at targets on the ground, but any shooting is better than none.

Just talking about this is making me want to get out there and shoot some real fish.