How to lower IDC?

It's important to keep IDC within an acceptable range (about 80%) so that when there is a sudden increase in airflow (e.g. on a cold day), one does not run lean suddenly and blow up the engine.

There's 4 ways to lower IDC:
1) lower your target boost above 5500rpm
2) lean out the afr above 5500rpm
3) mod your stock injectors
4) buy bigger injectors

Here's an interesting discussion on NASIOC.

I quote eggroll's comments which I think summarises it best:
The FPR will adjust fuel pressure to keep it 43 psi above the manifold pressure. The reason it does this is because the injectors are squirting fuel into, that's right, the manifold. If the fuel pressure remained at 43 psig, and the manifold pressure rose from 0 psig to 21.5 psig, even at the same IDC you would only be getting half the fuel, since the effective fuel pressure would only be 21.5 psig (43-21.5). This means that if there's a boost spike, it will only increase fuel pressure to keep a constant 43 psi differential between the fuel pressure and the manifold pressure, to allow your injectors to work as effectively at this higher pressure as they did at the lower pressure. This is completely separate from your injector potential, IDC, target AFR, everything (as long as it's working properly).

If you get a boost spike, your air flow will increase, which will cause your MAF voltage to increase. In turn the ECU will register that you have more airflow, and it will increase the IDC to add enough fuel to compensate. If your IDC cannot go any higher, you will get no extra fuel, and that extra air will cause a lean condition. Say you got a spike that increased airflow by 15% (not uncommon). If you are already at the injector limit, that 15% increase in airflow will cause your AFR to swing 15% more lean. If you were running at 11:1, you will swing to 12.65:1, add that to the fact that you're already running more boost than the car was tuned for, and you can see where the engine is headed...