Summoners
You have 4 options here; let's start with the mobility options:
Ghost vs. Flash - For ranged DPS, I have a huge preference to run Ghost, since sustained mobility is better for sustained dps (AD carries). Ez also has a built-in Flash on a 10s reducible CD, so the extra Flash is useful if you find yourself in a really bad situation. Also, ever since the Flash nerf, I've found I couldn't tower dive for the kill anyway anymore, so I've switched off to
Ignite - Ez shines the most in midgame, and Ignite is just the best way to capitalize on that. I run Ghost/Ignite myself, but I'm sure Flash/Ignite is strong too. I've found that before the Flash nerf, I'd have to tower dive and Flash away to survive to get a kill, but with Ignite, I never have to tower dive in the first place. The downside is it lacks the utility of Flash later on in the game, when you need it to survive team fights.
Cleanse - the option that I personally never run, but would do extremely well against certain matchups. Cleanse is only useful in ranked play when you know what you're up against because it has no utility against certain lineups, and massive utility in others. If you count 4-5 cleansable hard cc, cleanse is a good option.
Masteries
Currently I run 2/7/21. Key points are the ever-so-popular SoS, and the Utility tree is pretty obvious. The last 2 points you can throw wherever you want; I like to throw them into Critical Strike because I rightclick people a lot when I'm laning, and one crit almost guarantees a lane advantage.
Runes
Neo gives me shit for doing mixed runes, but I think my rune builds are superior to doing 1 stat/color. Here goes
Reds: Armor Pen (I think xpecial may do ad/lvl, but I'm not about to shell out the IP to test that out)
Yellows: Scaling Mp5s
Blues: 3 Scaling Mp5s, 6 Scaling MRs
Quints: 1-2 Flat AD, 1-2 Health Quints
This build lets you play pretty aggressively early in lane and scales well into late game. I only run enough Mp5s so that I have just enough mana to keep me running; mana is a stat that's only useful when you have none, so I only keep enough to keep from running dry. If you feel that your playstyle is more mana-hungry, then don't hesitate to adjust accordingly. The Flat AD/Health Quint mixture is to give you an edge over your lane opponent. Just a note: Flat HP/AD don't scale well at all, so feel free to run Armor Pen instead if you don't feel confident in your laning skills.
Item Build
The way I choose how to build items can be pretty complicated. I'll lay it all out for you for the tryhards out there, but if you're not that tryhard, just do Blade --> Boots --> Sheen --> CDR Boots --> Bloodthirster --> Trinity --> LW.
The detailed description:
1. Open Doran's Blade, unless I'm facing a Panth or strong AD laner + Shaco, in which case I open Shield.
2. Boots
3. Depending on how well I've farmed, I'll either get a Sheen or another Doran's Blade and a Long Sword. The main purpose of buying Sheen is that imba passive, so if I don't have enough to buy it all at once, I'll delay it with another Blade + Long Sword, which gives me the hitting power to win my lane some more (or lose less if I'm losing).
4. CDR Boots, 95% of the time. It's better to avoid getting CCed using positioning rather than relying on merc treads. CDR Boots let you throw out much more damage anyway, since Ez's dmg isn't as reliant on rightclicks.
5. Bloodthirster. The best item for flat AD. Lifesteal is also valid defense when you don't plan on getting mobbed by 5 people at once.
6. Finish Trinity - better proc, higher mobility, a slow when you don't have red. What's not to like?
After that, it's really situational depending on what occurs in a fight.
Getting crushed by their AP carry? Banshees
Getting jumped on by their WW/Malz? QSS
Not throwing out enough damage because their tanks are farmed? LW
Team worthless if you die first? GA
Skill Build
R>Q>E>W. Some ppl like to get an early point of W, but I find it mana-inefficient and it's only real advantage is its utility in teamfights and for adding another stack to your passive. I prefer getting E for the decreased CD, amplified when you land all your Qs so that it will have extremely short CD.
Playstyle
In lane, you'll have to judge for yourself whether you can play aggressively. Don't forget to factor in your ignite when estimating who has the upper hand. But really, Ezreal can play both passive or aggressive and still do well later on. Q is good for passively farming or harassing.
If you're farming well/got in an early kill, you can gank as soon as you hit Sheen. Otherwise, keep farming until you have a BF sword at least because Ezreal hits like a wimp if he's underfarmed.
In team fights, if their tanks are low mobility or underfarmed, you can just keep poking at them until they get a bit low. If they're too tanky for you to do any serious damage, then you'll have to rely on flanking, which is a risky maneuver but can pay off. Go around and try to find an opening to E onto their carry while not drawing much attention from the rest of their team. This isn't really optimal, but can be really effective if their tanks are really buff.
Also, try to open with your R so that you instantly have 5 stacks on your passive. Note that you can maintain/build stacks by using W on your teammates, while also increasing their attack speed to take down towers more quickly. Always try to maintain stacks, as Ezreal's passive is his steroid, and he still attacks relatively slow compared to some other carries.
So, that's all I can think of for now. If you have any questions, don't hesitate to ask here.
EDIT: Added more info on my rune choice because Neo qq at me.