Well this is my first actual build, but I've been playing Jarv since he came out. I'm not a particularly high ranked player, but I can safely say I can go toe to toe with any Jarv I've ever played against. The trick is finding a perfect mix of damage and health, because with the right build Jarv can be as invincible as Singed and yet do as much damage as say Xin Zhao. The truth is, Jarvan IV is a hidden gem. If you play him right he can rip apart any team with his initiation and then get out without a scratch with his shield.

Greater Seal of Desolation: Helps you penetrate more of your target's armor, seeing as you're completely AD except your Standard
Greater Mark of Scaling Magic Resist: Gives you more of that ever useful Magic Resistance mages are always crying about.
Greater Seal of Defense: Gives you more armor, so when Trynd starts wailing on you with his giant sword, you just might be able to kill him because he doesn't hurt you as much.
Greater Quintessence of Desolation: Again, more armor penetration.

Jarvan IV is ever dependant on the other team for his build. So say the enemy Tryndamere is doing amazing. Well that means you need a thorn mail, so what do you substitute? However, the things i never ever change are the first five items: Doran's Shield for some armor, hp and regain, mercury treads for the tenacity (reduces the time you're affected by slows, stuns, taunts, etc.), Heart of Gold for the nice hp and gold, and Phage for hp, damage, and the occasional slow, which leads to Trinity Force. Always get a trin. It can either make your game much better, or make your already good game a massacre. Jarvan IV is one of those champs that just makes trinity hurt, kinda like Poppy. Sheen gives you the mana to keep doing your pull, and when you get in the middle of a fight and start using abilities, it combines very well with your passive. Zeal is also effective on jarv. Alright imagine you have say 150 AD and you hit a first hit, with sheen's active, and a critical strike. Thats some serious damage. Anyway the phage also has that handy slow with your hp and damage.
Your next items depend on the other team. My IDEAL build is Mercury's Treads, Trinity Force, Randuin's Omen, Force of Nature, Frozen Mallet, and Atma's Impaler. But lets say that the enemy's Malzahar is doing amazing. Well by all means substitute the Force of Nature with a Banshees Veil. Or what if Tryndamere is destroying your team? Again, build either Thornmail or Frozen Heart. Or if you just like to waste mana like crazy, get the Manamune and Guardian Angel instead of the Frozen Mallet/ Atmas Impaler. Jarv's build is completely up to the combination of your style and the enemy's team composition. As to the second build, I almost never get it finished. The last item, which I have as warmogs, is completely situational. Pick whatever helps.

I always like to get my pull ASAP. I start with the Demacian Standard because it give extra armor and attack speed. Then level two get Dragon Strike to complete the pull ombo. You can do alot of good harass against squishies because it drops a quarter to a third of their hp every time you land it on them. A tip is that since the Standard is an AOE throw it behind them not on them because it still does the damage, and its easier to land the pop-up.
Anyway, back to skill sequence. I try to get my full combo of q,w, and e as soon as I can. This is weird for some people who like to get one powerful ability, but whatever. Once you have everything, start leveling up the Standard until level 6. Another tip: don't start poping out a full q,w,e combo at level 3,4, or 5. It will kill your mana, bring it down to about 50% because they cost alot and you don't have that much mana early. Save your combos and harass with your passive, and every once in a while use your q,e combo. Anyway I always get the Standard up pretty fast because it gives you a tiny bit more beef and AS. Somewhere about level 7, 8, or 9 I get another level of the Dragon Srtike for the cooldown and extra damage. The rest of the abilities go as above, because I have a more dps outlook. I mean you're rushing into the middle of the other team- you need all the armor you can and the Golden Aegis is good for not getting immediatly destroyed.

Ok i had to redo this section because I used to always used Ghost and Ignite/ Exhaust, but the truth is, as you get into ranked Flash is better. Although you have your pull, sometimes flash can help you to:
A. use your ult on the enemy carry
B. position yourself to pop the entire team
C. get the extra jump over walls where most champs cant follow you

Again if you didn't read the guide thouroghly, and because you need to be told again here are some tips:
YOU ARE DEPENDANt ON YOUR STYLE AND THE OTHER TEAM'S COMPOSITION!!! I cannot stress this enough! If you try to build the same for a team with all AD and a team with all AP, then you WILL be met with failure! Unless your team covers for you, or the other team sucks, thats how it goes.
CONSERVE YOUR MANA!!! You can't start blasting off the famous Jarv combos at level 3! That's not how it works! Harass with your passive, and the q,e combo.
YOU ARE A DPS SO YOU AREN'T INVINCIBLE!!! When you get your Trinity Force, DON'T make the mistake of thinking you can't be hurt! You will hurt very, very badly, but don't try to be the focus of attention... yet.

I've played this guide for a few months before deciding that maybe I should put it on Mobafire. So yes, it is well tested, but that's for me. You have to decide how it works for YOU. So give it a few tries before you post bad things on here and thumbs down it because you have to work it to your style. If you keep trying with Frozen Mallet/Atmas Impaler and think well I hate this guide, try the other combo!
Another thing! DON'T TRY THIS GUIDE FIRST time JARVAN IV AND THINK IT WILL MAKE YOU AMAZING! IT WILL NOT!!!
But if you play him right, Jarv can be the champion you've been dreaming of. Hard to kill, great damage, and pretty fast with Trinity, and FoN. So good luck to all you Jarv fans, and have fun.