Keeping track of IPs to Usernames

I was wondering how people are tracking inside user activity. We currently use DHCP and I have various monitoring utilities watching the network; however, after a day goes by or so the username to IP mapping will be incorrect.

Do you just use static mappings?

it depends...

for web activity for instance most of the time just user single-sign on and keep track of the users activity within the proxy log

why not use static IPs, its good from a security point, though I dont know the number of your systems. Else protect your dhcp server adequate. when I use UNTANGLE I can track IPs

I think I could get away with static IPs. Maybe do DHCP reservations?

Not sure if this is what you looking for, but solarwinds has this free tool called IP address tracker:

www.solarwinds.com/products/freetools/ip_address_tracker/

Have a look at the demonstration video for it.