I SUCK at writing things so i want to keep it short but with enough info to give you a look in my nasus build, any questions and/or additions/advice is welcome ;)
Well, nasus... All I say is that he is actually a very strong champ, especially late-game.
I once saw someone write: WTF NASUS 1500 Q CRITS!?
I answered: why not? Nasus is very underestimated, and especially how i play him you get 1000+ q crits... He got mad!
Let's get started!
As i said: I SUCK at writing so don't flame me for not being a good writer pls :)

Simply for the masteries: It gives a good boost to crits, normal dmg, armor penetration and bonus dmg for finishing off plus a boost to the passive. It gives a defensive edge too, so works just fine for me ^^
Summoner spells: Exhaust: it just slows and gives a great chunk of debuff to the enemy, in a lot of situations! Ignite: it counters vamp or life steal, and deals a ton of dmg without even doing anything at all! So i use it in every game.
Why not flash or ghost? I already got CC skill and exhaust, so don't need the flash AND ghost, and besides, the zeal of tri force gives me bonus movement as well!
Heal: I have an awesome passive! no need for heal! As for the rest. I don't jungle, i don't need surge since i play ad... That's it for the summoner spells!
Runes: Armor penetration for increased damage, and if i happen to crit exactly that q, it deals tons of damage. Crit damage runes for that extra bonus: masteries+10% and some odd 10% again for the glyphs, combined with infinity edge it's a lot more damage!~
The attack damage runes for the extra damage, won't hurt ^^

I start with a cloth armor to reduce the basic damage of minions AND ad champs alike, i build it into ninja tabi for the small dodge and the extra armor. I immediately start building up trinity force, and after three recalls i either have phage or sheen, depending on how much damage i take when I'm in lane. If it's much i take Phage first, then Sheen, then Zeal. After that I hit for more damage and crit with infinity edge. I hit for frozen mallet or bloodthirster, depending on how much gold i have, or how much damage i take. If i go well i build bloodthirster first and then frozen mallet. I finish with atma's impaler for bonus damage...
I usually hit 400-450 dmg with that build if it's into the game for that long.

Naus has a few good pros:
Good farm with q for largely increased damage.
His ult works perfectly for the bonus health, damage, and actually gives a boost to health.
CC skill totally blocks some ad carries like yi, xin, tryndamere (if he plays with a lot of attack speed).
You can play him very agressive depending on the lane you're in
Life steal passive!

Cons:
Enemy champs might run away or focus you when you toggle your ult (OMG BIG DOG THING!!!)
Relies on good farm

It's easy: Take the brunt of damage for your squishier teammates like teemo, fiddle (if he plays ap>hp) and veigar etc. The 'w' skill absolutely slows and decreases enemies, so in lane you can pick the squishier and take him down quickly combined with champs like Nasus/Vayne, Nasus/Xin, Nasus/Morde etc...
There's not much to say about it elsewise, but any additions, suggestions and explaining is welcome, so I can put this build better...

Just kill those minions, try to last hit with q as much as possible for biggest effect! Since the cooldown on q is 3-5 seconds (counting down 5-3 the higher the skill becomes).
I usually get 300-400 bonus damage on my q every game that lasts long enough for everyone to get a good score (usually 30-40 mins). If you get higher, then you're good, but don't put everything into damage or you'll get fried too quickly!

As i said before: I'm not so much of a good writer, my grammar sucks and i can't explain things clearly... But i still hope to get a good feedback on this build to work it out ;)
I usually get 10+/max 5 deaths each game... My worst game ever was 4/3/16 just for getting stolen and killed before getting assists... You don't really need kills if you're a good farmer because the q dmg will only stack higher!
Hope this is an inspiration for ye guys and gals ^^
If anyone can get me better grammar or feedback you're welcome ^^