For a long time in the beginning this happened to me. I would be in the big ring and try to trim down one click and I'd drop it into the small ring. It takes a while to get a feel for the touch...you'll get used to giving it just the right amount of pressure.

That said, do make sure that you're actually shifted all the way up and not already in the trim position. You can tell by how much "give" the downshift lever has when you press it over (but don't shift). If you're all the way up and press the lever over lightly, it will travel a short distance before the point of resistance (just where the shift is about to happen if you press farther). If you're in the trim position and press it over lightly, it will travel a bit farther before you hit the resistance.