Improving Importance Sampling estimators for rare event probabilities requires sharp approximations of conditional densities. This is achieved for events En := (u(X1) + ... + u(Xn)) ∈ An where the summands are i.i.d. and En is a large or moderate deviation event. The approximation of the conditional density of the vector (X1, ..., X kn ) with respect to En on long runs, when kn/n → 1, is handled. The maximal value of kn compatible with a given accuracy is discussed; simulated results are presented, which enlight the gain of the present approach over classical IS schemes. Detailed algorithms are proposed.