Unfortunately there aren't a lot of shortcuts when drawing by hand

If you have a light table that would help, as you could get a set of trees that you're very happy with on a separate pice of paper, and trace out as you go (my light table, when I had one, was a 200 or 250 watt bulb underneath a storm window pane that was up on blocks)

if you're ambitious and have some good exacto knife blades you could even make a stencil from cardstock.

When I was drawing out the mountains on my last few maps (it was on the computer, but I was using the tablet and drawing them individually anyhow) was to have an example of some really nice mountain shapes I was happy with set up as a step by step process. I had that open in the second monitor next to where I was working, and I'd refer to that picture as I went. Doing trees shouldn't be too different.. set up a piece of paper where you draw some that you're happy with, as a step by step (outlines, then shade the outlines, then add shadows, or whatever - each step laid out) and keep it close as a reference while you move around.

What I was doing with my celtic art on paper, when not using the light table, was simply drawing the same thing out over and over and over until it was just reflex. Eventually you should get to the point where you're happy with them as you draw them out and don't need the reference material as often.

Good luck and happy callouses!