Hey there.
So I'm every now and then playing Nasus, and then I try to play him pretty tanky (rune-wise and mastery-wise, as far as I can do so with lvl 24). Every now and then my team wins, but I often have the feeling of not contributing that much.

So, the way I play him is:
As spells I take cleanse and ghost. Cleanse because it just always is useful when I play in a tanky way, and ghost so that the opponents can't get away that easily (or that I can get away more easily).

In matters of skills, I first take a point in that melee thingy, and then it's for me:
ult > aoe thingy > slow > that melee hit something.

Masteries: since I'm not 30 I put one point in ghost, and stuff the rest into the defensive thingy.

In matters of items I go first for chalice, for the magic resist and I have more mana available for the aoe thingy, to make money and harass opponents. Then I go for boots (mercury normally, sometimes tabi or swiftness). Next I get a sheen, then build up my triforce, then an aegis or banshees veil and then frozen heart or sunfire.

The way I play him I obviously try to last hit with the melee skill a lot, but how well that works depends on my opponents. Then I try to make money with the aoe thing and harass my opponents, and when they are getting low, I try to wither them, rush up with ghost, aoe in front of them and hope that my teammate helps me to kill them. Sometimes that works, sometimes doesn't. Then the game progresses, and I try on the one hand to kill large groups of creeps with my aoe for gold and help in team battles by running in the midst of the team during a fight, popping ult and hitting the squishies with my sheen and melee. And later on I can also kill towers every now and then.

Problems:
- somehow my lifedrain doesn't do that much. If I have two ranged opponent in my lane, I can try to harrass them with my aoe, but I can't really hit the mobs for lifegain or buff my melee skill. And even if I can hit mobs every now and then, I still don't seem to get any substantial amount of life back
- I don't seem to actually be able to do any damage. Yes I can slow, use my melee thing and do the aoe stuff, but still: it's like hitting with a soft rubber sword (or stuff). Without a teammates help I have problems surviving 1vs1 encounters
- group fights have three outcomes. 1) they all rush away in different directions when I pop my ult, and sometimes even with ghost and wither I have problems chasing at least one, when they flash around and whatnout. 2) they all concentrate on me, keep me stunned and despite cleanse and ult kill me before I can do stuff. 3) they keep hitting us randomly, then get low on health and we can kill them since they can't escape the ghost and wither and aoe. But 1) and 2) happen more often than 3)
- I don't really feel like a tank, in matters of health. Like, chalice gives no life (but I need the mana), boots give no life (but I need them anyway). So the first thing that gives some life is phage or aegis, but once I have one of those, the match is already running for a long time. When I play Singed I get tons of life anyway, and when I play Rammus I can pop my curl thingy and then can take a beating. But as Nasus? I don't feel like a tank. Sure I could rush Warmogs or something, but I need mana to make gold. And I need the boots. And sheen to hit at least a little bit. So dunno what to do about that.

Any advice would be most welcomed.

If you have the IP for mana regeneration runes/lvl 18 that would help, take it on both yellow and blue. That way you can pass on the chalice of harmony so you can get faster to you core items. This helped me more than anything else.

Go: Doran shield - boots - Sheen - Improved boots - Aegis.

This will give you fair amount of HP (Doran shield and Aegis), damage (sheen, some from aegis) and resistance (aegis and improved boots as it should be mercuries and sometimes ninja).

The way you play is off. Don't expect to get any kills early on. Your main objective is to max the AoE attack (called spirit fire from this point on) and farm up. Last hit quickly gain gold. The lifesteal is a bonus, don't see it as a way to regenarate, get a doran shield for the health regen.

Your job is not to carry. You let a tank initiate, your spirit fire is to REDUCE armor in a teamfight, you put this in a spot that would be a good spot to stand in normally (bottleneck spot). You press R and all of the sudden you drain health at an amazing speed and build up your attack damage. As you walk to an important target with a lot of health (doran shield, aegis, bonus from ultimate and high base health) you wither this carry (cannot get away, their DPS drop instantly thanks to attack speed slow) you hit them once, and then once more with siphoning strike. Your attack damage is insane thanks to ultimate, 2 sheen procs on top of that and 1 siphoning strike extra damage. All the while you are draining 5% of their HP. Now they can choose to attack you removing heat from teammates or let you be while you decimate them.

You support your team by your aura items and lowering their armor, use your crowd control to remove a lot of damage from physical dps as well as slow them down so they can be focused.

Outside of teamfights you are an excellent farmer that can push lanes in matter of seconds which is great to push after a teamfight. Especially later on your passive will grant you the ability to regen out of battle pretty fast without investing in life steal items.

Might be time to pick up nasus again, I miss the dawg =(

all your problems can be remedied by one thought. you're a tank, not a dps. go aegis -> merc treads -> trinity force and farm lots. if people focus you, they leave your teamates open, they take damage, and you'll be whacking them for 500 ss crits.

i can't stress this enough, you're a freakin tank, so play like one. dont go rampant though, save your ult for after initiation.

Triforce is an amazing item, but I don't think you really want to rush it. Better to get some tank items after sheen.