Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
It is a common misconception to build Caitlyn with straight damage. That is in fact, less efficient because RIOT happened to make her weak as balls. There is hardly any scaling on her ult or on her Q (peacemaker). However, there is a way to build Caitlyn so that she can carry, even in late game. We do this by exploiting her best ability---her passive.
It is helpful to have CD Reduction Runes along with Crit DMG or Crit Chance runes. I personally have about 33% Crit DMG, 8% CDR by lvl 18, and about 3% CDR right off the bat.
Since we'll be doing a nice DPS build on Caitlyn, go for purely physical DMG masteries. Keep the rest of your 8 points in Utility.
Items + Gameplay
I am assuming that you watched the champion spotlight about how to jump over walls, jump away from people, and set up good traps. As we proceed, just remember that Caitlyn is NOT an initiating champ. Let your team initiate and follow to sweep up.
So here is the serious item justification. How to exploit her passive:
--1st set of items:
Doran's Blade + Health Potion - nice DMG, HP, and Lifesteal to keep it going in the lane. A HP pot should keep you set until you have to go back.
FARM UP with your Q and attack from the bushes if you can. Use Clairvoyance to scare enemies out of the bush so that you can set up shop there. It is tough against heroes like nidalee or mundo, who can skill shot you in the bush anyways.
--2nd set of items:
Madred's Razors + Boots - At about 1350, go back and get yourself these items.
This will automatically make you farm SOOO much faster, and it will be easy to get red lizard buff now. At this point you do not have to last hit. Push the lane if you want, but make sure that you have Snap Traps in convenient locations (edges of bushes so you don't get ganked). Also you CV often to check enemy jungle for their jungler.
--3rd set of items:
Berserkers Greaves + Sword of Divine - By now you have serious attack speed and farming power.
Clairvoyance really helps will your ult. To make sure your target does not go out of sight range, cast your ult and then cast CV after. You should have some kills by now especially with these items. The magic damage from SoD gives a nice surprise to your enemies and the necessary armor penetration and straight damage helps you get kills for sure. Nobody can dodge your attacks when its critical!
--4th set of items:
Madred's BloodRazor -> Infinity Edge - By this point, you no longer need the 500 dmg to minions, so finish your bloodrazors so that you can kill anybody, even tanks.
Bloodrazor deals magic damage, so at this point you are doing decent Physical Damage and a **** ton of Magic Damage with your attacks. With infinity edge, there's no good way for the enemy team to really counter you. With both these items, watch the damage it shows on people...It will be a nonstop pillar of numbers. You should have red buff often as well.
You will be doing base damage, 100 mg dmg about every second from SoD, mg dmg from Madreds, the damage and slow from red buff, and the nice crit dmg from infinity edge. On top of all that, some attacks will be multiplied by 150% on champs, and 250% on minions...GODDAM!
Exploit that PASSIVE!!!!!
--5th set of items:
Phantom Dancer + Bloodthirster (or Frozen Mallet or Tiamat) - Phantom Dancer just adds more crazy crit and attack speed. This will seriously MOW-DOWN your enemies by exploiting your passive even HARDER.
As your last item, you have a choice between a BloodThirster, Mallet, or Tiamat. Tiamat is great for team battle because then you do damage to everybody. Mallet is good so you can single somebody out and kill them. Bloodthirster is helpful with more damage and survivability. It really depends on the opponent.
In the end, you actually hardly want to use your attack abilities on enemy champs unless you want to get kills that are out of your reach. Caitlyns passive makes her auto attacks monstrous. By the end of the match, you should have insane attack speed, doing so much damage that your computer can't keep up. It is also a mixture of Magic and Physical Damage, so you are not easily countered.
remember: CAITLYN IS NOT AN INITIATING CHAMP.
So, go forth. Finally own noobs with the new champ. Trust me, they won't be expecting this one.
-me AKA shaggi AKA Sorry4KillingYou