If a cracker cannot gain access to your system, then there is no way for them to install a sniffer onto it. In a perfect world, we would be able to stop here. But since there are an unprecedented number of security holes found each month and most companies don't have enough staff to fix these holes, then crackers are going to exploit vulnerabilities and install sniffers. A good way to protect your network against sniffers is to segment it as much as possible using Ethernet switches instead of regular hubs. The drawback to this solution is cost.
Switches are two to three times more expensive then hubs, but the trade-off is definitely worth it.
Another option, which you can combine with a switched environment, is to use encryption. The sniffer still sees the traffic, but it is displayed as garbled data. Some drawbacks of using encryption are the speed and the chance of you using a weak encryption standard that can be easily broken. Almost all encryption will introduce delay into your network. Typically, the stronger the encryption, the slower the machines using it will communicate.