When marking music, here are some of the essential shorthand Italian (and one French) terms I love to use:

• rit. (short for ritard), meaning to gradually slow down
• a tempo, meaning back to the original tempo
• v.s. (short for volta subito), meaning to turn the page quickly
• restez (French for stay), meaning to stay in position (i.e. not shift down); isn't it so much cooler to know how to use that term, rather than just writing "stay" in English? It makes me feel more refined, lol.
• accel. (short for accelerando), meaning to gradually get faster
• al niente, meaning to fade to nothing
• sul tasto, meaning "over the fingerborad" (but just slightly, on the edge of the black)
• dolcissimo, meaning very sweetly
• stringendo, interchangeable with accelerando (it just depends on which one I'm in the mood for)
• senza sord. (short for senza sordino), meaning to take off your mute
• soli, meaning a section solo and we as 1st or 2nd violins should play a little louder
• I, II, III, IV to indicate which string to be on, often next to a certain fingering (okay, okay, not exactly Italian . . . but important!)
• poco a poco, meaning little by little; can be used, for example, in conjunction with terms like "stringendo"
• meno, meaning less
• simile, meaning the same (I usually use this when I don't want to write out 30 slurs in an extended passage of similarly patterned notes; it indicates to continue the same bowing pattern--mark the bowing itself twice, and above the third iteration, simply write "simile")
• 8vb (short for ottava bassa); most violinists are used to seeing 8va, but there's a counterpart: 8vb means an octave lower
• Note: There are a couple of shorthand symbols I also like, but only when writing the Italian would be too lengthy: a triangle, for a conducting pattern of 3, eyeglasses to denote looking at the conductor, and a backwards arrow to indicate holding back the tempo without a noticeable change in speed. Other players will use additional symbols, such as a squiggly line to denote slowing down, but I love Italian, so I prefer to quickly write rit.

Well, that does it! Enjoy incorporating more of these in your music notation!