Xbee-Arduino serial input (outliers problem)
According to the Arduino Reference pulseIn: "Works on pulses from 10 microseconds to 3 minutes in length." So that should be borne in mind.

To answer your question, my understanding is that you want to "clean up" or filter the rssiDur value. You could filter this digitally using a low pass filter to approximate the mean value:

int rssiDurFilt = (1-0.99)*rssiDur + 0.99*rssiDurFilt;


Change the 0.99 to change the cut off frequency (closer to 1.0 is lower frequency). The actual expression for that value is exp(-2*pi*f/fs) where f is the cutoff frequency you want and fs is the frequency the data is sampled at.

Another type of "digital filter" is an event filter. It works well on data that has outliers; e.g. 9,9,8,10,9,25,9  In this case you fill an array with sampled values and build a frequency table (i.e. count the number of occurrences of each value), then select the value with the most occurrences. Statistically this is the mode.

UPDATE:
Statistical averages such as Mean, Mode etc.. can be calculated using the Arduino Average Library.