I believe Nami is one of the most overpowered utility supports at the moment. Not only does she bring a powerful presence to any team fight from level 1 invades to endgame, she is actually enjoyable to play support with.

In this guide, I will try to break down her mechanics as clearly as possible. Hopefully, after reading it, you will have a better understanding of Nami and get some wins. The guide is still under construction, so it will be updated later. Thanks!

Pros
+High CC
+Good Poke
+Stems Mistakes of Careless Carries
+Gives a Red Buffesque Slow to Carries (e-ability)
+Movement Speed Buff on Spell Casts to Ally Target


Cons:
-Slightly Difficult Skill Shots
-TeamWork Dependent (harder to play in SoloQ
-Ultimate Will not go off when it is cast immediately before dying

Standard Support Page -Experimental Support Nami Page - Focus on Early Harrass and Sustain

Starting with

Ancient Coin gives +5h + 3m
Base regens on Nami are +4.5h +6.9m
Thus lvl 1 you have +12.5hp5 and +12.9mp5.
At lvl 2 the regen is slightly increased to 13.05hp5 and 13.5mp5 which in theory allows you to use Q(60 mana/appr. 13second C/D with runes/masteries)=>W(70 mana/appr. 8 second C/D with runes/masteries)Combo(total mana cost 130) more often to harass, or kill the enemy.
For example, -as a bench mark, in a perfect world- in 1 minute you could use this combo max 4 times at a cost of 520 mana, but with without the mastery Nami's mana at lvl 2 (348) with a regen of items and solely herself you only get an additional +136 mana, thus total of 484. with the masteries you gain an additional 36 per minute giving you 510. Thus, if you were to constantly harass as soon as abilities are up, in approximately 1 minute you could put out 580 damage and heal for 560 as opposed to 505/495. To me this makes the points in these masteries cost effective and efficient.

Note: I did not put a point in wanderer, because the focus is harass, not being passive (which would make wanderer more valuable). I chose not to put a point in bandit because there are other more cost effective masteries at earlier levels like the CDR in offense.



Helpful Gold Value Chart by author AetherThought on Reddit: http://www.reddit.com/r/leagueoflegends/comments/1rkvad/calculating_gold_efficiency_of_masteries/

I Initially had gold per 10 quints, but have revamped my rune page for season 4. They are a little experimental, but this has been working out fairly decent.

Season 3 - Gold per 10
Season 4 - Focus Harass


Quints
2 Greater Quint of Move Speed - MS is incredibly important in the current meta.
1 Greater Quint Ability Power

Seals:
8 Greater Seal of Armor

Marks:
4 Greater Mark of Scaling Ability Power
5 Greater Mark of Armor

Glyphs:
2 Greater Glyph of Cooldown Reduction - Experimental, but early game cooldowns are higher, so I haven't found a reason to not have them.
7 Greater Glyph of Magic Resist


Result:
5.42 AP at lvl 1/ 12.47 AP at lvl 18
17.2 Armor
9.4 Magic Resist
3% Movement Speed
1.67% Cooldown Reduction

Surging Tides Nami's Passive allows for early mistakes from carries who may engage too hard, or yourself if you are being overly aggressive and get caught. When Nami's abilities hit allied champions they gain 40 (+0.1 per ability power) movement speed for 1.5 seconds.

Aqua PrisonNami's Bubble. I know you all have had those unfortunate games as a carry, being supported by a Nami who cannot land a single Q. I am sorry, for your lose... But, with Nami's 1.5 pop up stunlock, the high risk is worth it. It is perfect for engaging and disengaging. Level 1 invades = kill secured.

Ebb and Flow Nami's Heal. This should be maxed first. It will provide so much lane sustain for your carry that he will never need to B. I also like Ebb and Flow because it can be used aggressively, and it still will heal you or your carry, so long as they are in range of the bounce back.Don't forget that the bounce range is further (875) then the cast range (725)

Tidecaller's Blessing Nami's secret super ability. I love Nami's E, I always remind my carries at the beginning of the game that when I cast E onto them they will receive a Slow, a Movement Speed Buff, and an Attack Buff. Be stingy with casting this, you don't want to run out of mana. You can use it as a gap closer later in the game, and a disengage if you are caught out while warding. Cast it leaving base to get that extra move speed when leaving.

Tidal Wave Nami's knockup. The range and width on this is just too OP. Use this to engage and disengage.

Starting Items

I start with Ancient Coin. I believe it is best on Nami who will need the CD reduction of Talisman of Ascension later on. I also believe it positively reinforces staying with your ADC at all times, because you will not get the bonus gold unless near them as they kill minions. (clarification: you do get the gold if your carry doesn't kill the minion or the carry is not nearby, I meant while in lane, if you stay too far behind your carry or away you don't get the gold because of the radius you have to be within the minion kill)

First Back

I prefer going Nomad's Medallion before boots, partly because if I truly need the movement speed I will buff myself with my passive [surging tides].
Normally, I do not rush Sightstone. With trinkets, as long as your ADC is also warding, you should have enough vision if you buy 1 or 2 Stealth Ward on your first backs. Buying a pink Vision Ward instead of stealth is also viable.

Core Items

Complete your [Talisman of Ascension] first. I like to get the cool-down reduction as soon as possible. Note: Do not complete your Talisman however unless you can buy wards.

Buy your [sightstone] next, you probably will be out or near out of laning phase by this time anyways.

You also want to start buying items to complete Seeker's Armguard. I usually start with Amplifying Tome.

For Boots, I prefer buying Mercury's Treads. In my opinion, tenacity will be more beneficial than the CDR from Ionian Boots of Lucidity. Especially if you get Talisman of Ascension and then Athene's Unholy Grail later.

Final Build

After boot upgrades, complete your Zhonya's Hourglass Zhonya's Hourglass.

Near end game you should be able to buy Athene's Athene's Unholy Grail for completed CDR, more magic resist, and ability scaling with AP. Buy Chalice Chalice of Harmony first. Because you may only place 3 wards on the map at one time, do not upgrade your sight stone into a ruby sightstone Ruby Sightstone, until you have full build. It is unnecessary and not gold efficient, for the health received.

Attack Combo Pre-Six
Q > E your carry > W enemy

Attack Combo Six+
R > Q > E your carry > W enemy

I suggest using R first because the knockup will make it easier to hit your Q.

Engaging While Near or Under Tower
Q > E your carry ... wait to see if kill is possible. If yes, auto attack > W carry if he is under turret.

The main difference when engaging near a tower is you want to hold on to your W to prevent a death of either you or your carry. If it is a complete flop engage, use R to hard disengage.

For Disengage:
Q > E yourself > Auto Attack to hit Slow > W yourself to maintain speed boost to disengage.

Another Combo Option
IF enemy is getting away Flash > Q
Its risky, and I personally do not need Flash to disengage usually, and you will not either if you play Nami properly!

A few final notes: only play as aggressive as your ADC. Roaming Nami to Mid could be game changing. It is a risk, since you are losing out on gold and leaving your ADC solo, but also has potential to be very rewarding. If you secure a double kill bottom-lane then roam mid. When roaming mid, sit in bush E your self and auto attack enemy. > Q > AA > W. Kill should be secured.

I hope this guide has helped. I am still working on it, so there will be updates in the future. If you have any Questions or Suggestions, feel free to discuss them with me.