The easiest and most reliable way that I have found is to run a cable from the headphone jack on the scanner to the "line-in" on your computer then use software to record the traffic. I used to have a program that recorded only when there was traffic so I could listen to alot of traffic in a short amount of time.
I'll try and find it and post it here for you. Its free, I think its called scanner recorder or something like that. Its a great program. I heard an entire OWI arrest in 10 minutes that took nearly 1.5 hours in real time...lol
I found the software here:
http://www.davee.com/scanrec/
Rob