You need:
a compass (or knowledge of directions in your area!)
the correct time - vital.
Before you try to spot one - go to
http://www.greenwichmeantime.com
to set your time - iridiums
always run on time, generally within several seconds - differences probably due to differences in what lat/long you are using and what lat/long you are standing on.
Then try again - depending on the brightness and how dark it is, they are pretty cool.
Once they are brighter than about -3 they are really easy - the westerly ones are harder if there is still light from the sunset of course. My 62 year old mum spotted saw a -4 from suburban Brisbane, having the correct time is the clincher. I've driven out to the lat/long of a -8 flare once, in a very dark country location - spectacular!
I usually get outside a minute or two early and catch them at least 95% of the time if it is perfectly clear. If you have the right time, call it off after a minute after the time.... you've missed the bus!
In dark sites you can just see them coming before they flare also, not in the city though.