I've been trying to practicing my last hitting vs bots because my cs does not live up to my expectations. Right now I get about 73 cs per game against bots with the occasional game with 80+ cs. After about 18 ten minute games, my cs is starting to drop down to an average of 64 per game.

My goal is a consistent 90+ cs so I'm open to suggestions of how to improve.... (so I can focus more on when the enemy needs to last hit)

(I already know about setting up minions and how they target, but I can't say I can apply the knowledge very well)

Edit: Back to 73-74 a game. It always seems the same for some reason.

Edit2: I'm only bad at cs during the first 10 minutes, especially with Tristana (Before my E is maxed)

Get more Ad and less AS/Crit/Penetration.
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Without knowing your champions its hard to help

You just said you don't buy any items in these practice runs, which means your damage is going nearly as high as it should be, last hitting especially with trist and ashe gets a lot easier with items because early game they both have very low AD, even with masteries and whatnot compared to other AD carries.

If you're practicing you should always buy items because last hitting with items is going to be much more lenient than without, and you could potentially miss CS waiting too long for a minion to get low when you have low AD. (Also especially true of a low level trist since she also has to get quite close to her target) More importantly you should always practice assuming 'combat conditions' basically assuming you're using what you'd use in a serious match. In a real game you're going to have to go back eventually. If you don't know when the right time to go back is or wait too long you can screw yourself by hanging around and getting killed by someone who does go buy.

Minions don't have much armor so once you hit 90-100 AD last hitting them becomes pretty easy.Alternatively you could try using a different champion one with a skill you can use to last hit with, for example pantheon can spear a minion for last hits if he really needs to. Or Fiora can Lunge. Ashe can use her volley for clearing a wave(though I wouldn't recommend that early on)

I should clarify, I don't go back to buy items. I usually start with boots and health potions.

The reason I don't go back is so I can have an accurate measurement of how much I've improved. Every time I look up how to improve last hitting it says to play a 10 minute match and try to aim for 100 cs, rinse and repeat till it becomes a habit rather than a task. That is what I'm doing.

Were those 18 ten-minute games one after another, or spaced out over the course of a few days? If it was in one block, then that would've been 3 straight hours of nonstop csing; you're probably just getting tired. Take a break, get a good night's sleep, I'm sure you'll see the fruits of your labor if you try it again tomorrow.

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.

@BadPlayer6
Yes they were consecutive games. You get a 3 minute break starting up the next game.

@tawfvxfve
My biggest problems are when I have 7+ magic minions attacking the same target at the same time. I don't know how to prevent that from happening. Another issue is when the siege minion is attacking one target while a large group of normal minions focus another. If I miss one, I end up missing a few in a row.

If you want, I can add / spectate you to see exactly how you're doing.

Yeah, you were probably just mentally exhausted xP Try again tomorrow, and see how you fare.

When you have a ton of caster minions attacking the same target, and the target has enough health that 1 of your autos won't kill it but one more round of caster minion attacks will, I just try to attack it so that my attack hits it right in the middle of the caster minion attacks (the exact timing depends on the individual champ's aa animation speed and aa projectile speed). It works more often than not, at least for me.

Not completely sure what you mean by the second situation, though... If it looks like two minions are going to die at the same time, you can attack one once early so that you'll be able to last hit both.