What I know helps when I'm trail breaking my horses is to make what I'm asking them to do (i.e. crossing puddles or shadows) the least amount of work. So, say, Greta won't cross a VERY small ditch. She walks up to it and balks, and dances, and turns away from it. In order to make her realize that she can't get away with a refusal like that, I do one of two things. The first thing I'll try is taking her in tight circles right next to the ditch. Then I stop her, and ask her again. This will usually do the trick, even if it does take a few rounds of circles. She soon will figure out that it's WAY easier just to walk over the ditch.
Another trick I use (which will also take a bit of repetition) is to turn her 180 degrees from the offending piece of trail (ha ha) and lope her straight away from it (provided the ground is flat for a good stretch.) Then I stop her, turn her back toward the ditch, and give her the reins and ask her to walk. This allows her to have her head, relax after loping. When we get back up to the place where she balked, she'll more than likely balk again, and not want to cross. I'll turn her around again and lope her again, no hesitation. Then we walk back, slow and collected. She'll eventually associate balking and refusing with working - and again, think that crossing is a much simpler solution.
I hope this helps!