Sharon,

Usually, the answer to this question would be "as soon as possible" or "at least one month before the move date". In reality however it really depends on several factors, such as:

1. Are you moving in the summer?
2. Are you moving in the last few or first few days of the month?
3. Do you plan to hire a small company?
4. Etc.

If you are moving in the summer or in the end of the month then it is a good idea to book your move not less then 2 weeks before the move date, and preferably even earlier.  

Other than that, it is usually not that difficult to book even few days before the actual move (there isn't really much for the moving company to prepare for your move, so if they are not that busy they can even accommodate your needs on a single day notice). 

As a rule of thumb, however, do not wait for too long to book your job.