Get AD masteries and flat AD runes. Also, what helped me is looking at your damage on your character sheet, then selecting the minion getting damaged and hit him when his HP falls below your threshold. Example : My AD as Ashe is 73. Click on minion, his info sheet pops up, his health is at 200 and falling, once it hits 65, last hit.
Counting health helped me immensely. Once you get the hang of that, it'll become intuitive just by looking at health bars.
Characters like Kog, Ashe, and Cait can also use their abilities to catch a minion that's about to drop when your auto attack is resetting from another minion kill. If you watch any high level ADC streams, you'll see the pros do this constantly so they don't miss any CS.
Another tip, if you can't harass and CS at the same time, don't start out trying. Just focus on CS.