There are MANY ways to do this!

-Remove Black Ops Update.(XBOX)
1. Go to hard-drive or the storage device you put the update on.
2. Click games, Black Ops.
3. Click Black Ops Title Update.
4. Click Delete.
5. Launch Black Ops and Re-Update.

-(PS3)
1. Game Data Utility.
2. Black Ops.
3. Delete.

If that doesn't work, Try this.

- Limit Bandwidth
1. Make sure you have no browsers or downloads running while playing.
Still lagging, Try this.

-Turn Router or Modem off.
1. Turn the Modem off and unplug the power cable.
Same with router.
2. Then plug every thing back in.

-Try port forwarding
1. Go to Portforward.com
(IF your using a Router some games you need to open up the ports!)
XBOX LIVE:
53
88
3074
PSN:
TCP 80
TCP 443
TCP 5223
UDP 3478 -3479
UDP 3658

-Change Servers
1. Go to in game party.
2. Press the back button.
3. Scrool to you see default left click till it says "LOCAL" or "LOCAL ONLY"

Still suck all I can say is Change your ISP(Internet Service) Provider

NOTE* Black Ops Isnt a sever its the game hosts sever so lets say you pay for 20MB'S a month and the host pays for 10MB'S per month You will only get 10MB and maybe even less!