Caitlyn is an awesome hero to play; with 650 range you can harass so well that people just crumble early game. Her midgame is totally team dependent due to her inability to survive cc fights and she lacks lategame utility apart from her traps and ult - so ideally she is best for early game pushes. Most builds you would have seen either rush for attack speed (via madreds) or attack damage (via bloodthirster). I find the best way to play her is by getting max armor penetration early game e.g. with or , thereby letting you really hurt through ur harassment and Q ability . After u get ur attack damage/armor pen, you really need to tank her up via banshees (against those insane burst casters like kass) or via frozen mallet (against heavy attack-oriented characters like xin).

Typical 21/0/9 with Flash and Exhaust . I used to use ghost cleanse but i find flash more effect for early game kills and escapes, plus the new exhaust is great for stopping high dps champs or for reducing burst output from mages.

For runes, i like to use armor pen redsGreater Mark of Desolation/quintsGreater Quintessence of Desolation and mana regen per 5 per level for yellows [icon-Greater Seal of Clarity size=30] and blues .

For skills, i like to start with 1 point in traps if u are laning then it helps get first blood and secure ur bush; otherwise get ur first point in blade (Q) . Maxing out Q is ur best bet for damage, followed by ur traps and then net - whilst maxing ur ult at lvls 6/11/16.

--> lets u move very fast for sniping opponents when u use passive. I find this better than atk speed since u will not have much chance of standing there and autoattacking; rather, you should be continually nagging ur opponents with ur awesome 650 range!
--> gives u lifesteal for laning and also crit chance :)
--> mainly to give u some health and block those annoying stuns (that usually result in ur death).
--> if the game lasts this long then this a great item to boost her damage.

Here are a few pointers:
1) throw down ur traps in the corners of the bush, especially in side lanes where they would enter the bush and sometimes in the corner from the river (near dragon/golem) if they hav junglers.
2) harass ur opponents with one or two attacks to leech their health down; this is pivotal early game since u do major damage with ur uber range and armor pen.
3) if enemies are standing still, or u hav a clear shot of them without too many minions in the way, use ur blade (Q) shot at them - it does a lot of damage if it just hits the champs.
4) learn when to use ur ult; always be mindful if it can be intercepted by another player. Sometimes if im 1v1 i like to use my ult on full health champs to greatly lower their health and keep them on the defensive play (since i can bring my ult to about 55 second cd).
5) use ur net shot to make some great escapes through thin walls or over river walls (e.g. through dragon walls). Sometimes if an enemy is ghosting away but u know ur allies can slow them if they hav a clear shot, it is better to use ur net to slow the opponent. Even though u wil be out of range for an attack, ur allies may be able to catchup and finish this kill (or even just give u line of sight so u can channel ur ult).