Keep some chunks loaded in Minecraft SMP

If you're using Bukkit you can use Forever ALoad, where you can select regions of map to keep permanently loaded.

Once installed it has a very simple chat interface:

/chunk        - Tells you how many chunks you will have (at a maximum) 
                remaining from unloading, and give stats about it

/chunk add    - Adds the chunk you're on to the Do-Not-Unload list

/chunk remove - Removes the chunk from the list

/chunk check  - Checks whether the chunk can unload

Minecraft 1.13.1 and newer versions have a /forceload command:

/forceload add x z

Where x and z are the coordinates of the chunk you want to load (specified as number of chunks from 0,0), not the number of blocks