When I was just starting to broaden my ears to the brilliance of Domenico Scarlatti’s Sonatas, I heard one of my early teachers, Robert Squires, play through Scarlatti’s Sonata in A Minor, K.54 (L.241). He played all of the trills cross-stringed and his reasoning was that a harpsichord would also perform the ornaments cross-stringed. Whatever the reason, it sounded so wonderful to hear the crispness and clarity of the trills played this way. Since then, I have worked on a lot of sonatas and have found that regularly practicing the following right hand formulas really help to develop and maintain this skill. There is a lot of beauty in playing trills with slurs but in a lot of baroque keyboard music, performing trills and ornaments across strings is worth the work.