After a visit and lesson from a builder, I have changed my mortaring technique and my mixture. He suggested using equal amounts cement and lime, to make the mixture even more workable, and also told me to add more parts of sand, since mortar should always be weaker than the bricks, and these bricks are pretty weak to begin with.
I also found out that the mix I was using was way too dry, which would explain why it wasn't sticking well to the bricks. Whereas before I was working with a very hard, crumbly mortar (sort of like natural peanut butter), his mix was soft and very runny, like warm butter. (Side note: I keep finding ways in which bricks and bread are similar. In the ways that they are mixed from two main ingredients, formed, baked, and even buttered! When the kiln is firing, it even gives off a moist, dough-like aroma)
The builder shows me how to hollow out the bed so that it has somewhere to 'squish' when I put the brick down. Since the mortar is so wet when it is first applied, he suggested that I lay a whole course of bricks, then go back and clean up the joints, because by then the clay will have soaked up a lot of the water.
These are his tools- a huge trowel, a small trowel, and his personal jointing bar, which was the handle of an old milk churn.
He was 'baffled' by the shape of my bricks, so he demonstrated the proper use of the jointing bar on the field wall. Basically, it's important to compress the mortar into the joint to create a clean, weatherproof finish.