DS strugles with handwirting. In OT they taped paper to the wall and wirte the letters vertically. They also told him to always warm up his finger muscles with silly putty before writing. He is supposed to use just his thumb and pointer finger to break of a small (pea sized) piece of putty and try to form it into a cube. Sometimes he just "plays" making cubes when he is bored and it does seem to have helped his writing come along.
Simply playing with other fine motor skill toys is always good too (this is the former preschool teacher in me comming out). Good ets are stringing beads, legos, using tweezers to move items from one container to another, he plastic beads you arrange and melt together (sorry the name escapes me at the moment), play dough, weaving looms, cutting an pasting, etc. This is the kind of stuff that gets him ready to be able to write letters.
Fun general letter making activities include: "paint" big letters on the sidewalk with a brush dipped in water, write in the sand at the beach with sticks or fingers, role play dough "snakes" and twist those into letters, make letter shaped sugar cookies for him to frost, try to make your body look like different letters (think the YMCA dance), sidewalk chalk, cover a table with shaving cream and trace letters into the cream with a finger, finger paint big letters, etc.