I've been messing with my Caitlyn build since she came out, and I think this is the best I've been able to produce so far. I tend to a more conservative play style when it comes to using skills, and so this build will suit players who use their skills sparingly. It's most effective if you try to last hit minions rather than just auto-attacking indiscriminately, and use your skills only to harass the enemy champion. You should be doing effectively true damage to almost anyone you will face in the middle lane early on, so you can you Caitlyn's long range and passive to harass without dipping into your mana pool to use skills, except when you think you can get the kill. Piltover Peacemaker will do significant damage early on, and will continue to be an effective skill until late game, when you will have to mostly rely on Cait's auto attack and significant damage to be effective.

I set Caitlyn's runes up to be a pure offensive set. I use flat Strength Seals, Glyphs, and Quintessences, with 1 Mark of Strength and 8 Marks of Desolation(The Might Seals above should be Seals of Strength, but for some reason the Strength Seals aren't listed in the options). The Armor penetration marks combined with the offensive masteries gives almost 20 armor penetration at level 1, which will deal true or nearly true damage to most champions you will mid against at level 1. The flat strength in all other slots combined with the offensive masteries will grant approximately 20 damage to Cait at level one as well. Use skills sparingly since there are no runes affecting mana.

I used a normal 21/0/9 build on Cait, with the main points being the armor penetration, attack speed, and damage masteries, and getting the %15 neutral monster buff from utility. Another good build is to use 15/0/15, to max the neutral buff and get the flash mastery.

My item build is somewhat situational, with the staples being bloodthirster, infinity edge, and phantom dancer. I like to start with doran's blade, and with my first return i'll get boots of speed and potions. On subsequent returns I generally try to build in the order I have the items above, although some situations require tweaks to the order. Always focus on damage first, because Cait's abilities scale so that they gain more than her total damage, which is incredibly useful early.

I level up Piltover Peacemaker to 5 first, with 1 point in Yordle Snap Trap and .90 Caliber Net until level 6. After Peacemaker I level the net and then last the trap, taking her ultimate whenever I can. Her ultimate is best used as a finisher if you think the enemy might get away, since once she starts channeling it will always fire(unless the channel is broken by a stun, toss, or taunt) even if they go out of range or you lose line of sight. She has to do the full animation to get into the channel though, if you lose sight before the channel actually starts, it'll break but still cost the mp.

I like to take exhaust and flash as my main summoner spells. Exhaust is useful early as either an escape or to pursue a fleeing opponent, and you can also use it it even up a fight if your taking the worst of it, since it has %70 physical damage reduction and %35 magic and item dmg reduction. Flash kinda speaks for itself.
Other good skills include Ghost and Cleanse, which will help if you expect to face ganking often.

Again, this build is for players who like to play conservatively with their skills. Only use your skills when you really need to, or you will face mana starvation early in the game. Harass as much as you can with Headshot and Caitlyn's long range rather than her trap or Peacemaker, although early on Peacemaker will do significant damage to most champions you'll find yourself facing in the middle lane. Try to last hit minions rather than farm them outright, you can reduce the chances of being ganked by not pushing their tower too early and exposing yourself. Unless your opponent likes to stack armor early on, you'll do true damage or nearly that every hit up until level 10 or so, when your items will compensate for their increased armor.

This build has not been tested in ranked play(by me at least). If you have any comments, please let me know!

When you reach late game, Cait's damage spells become largely useless. Piltover Peacemaker simply won't do enough damage past the armor to be worthwhile against most enemies. Her other skills are useful only in an indirect manner. .90 Caliber Net can pretty much only be used as an escape, while her Yordle Snap Trap can be good to place in the middle of a team fight to immobilize an enemy champion in the heat of battle. Always use Ace in the Hole only as a finisher, it's long range and perfect accuracy can prevent an escaping enemy from getting away.

Try to keep yourself away from the fight at first, if you come in after it's initiated, you're less likely to be a target, and you can do incredible damage when you're not under attack. Place a trap in the middle of the maelstrom, and place more if the fight is still going when the cooldown finishes. If a low-health enemy is trying to escape, you can ult them if you can't catch up, but don't get separated from your team; if you get caught out alone, you're pretty much dead. Cait simply doesn't have the survivability necessary to handle a late game gank.