Use a decent timer, like nunof says. Either use a decent, high load timer, or a cheap timer plus a relay (aka contactor). If you use a relay, you will not have to worry about timer lockups, since the load will not go through the timer directly.
Be careful with the rating on timers, especially if you use a magnetic ballast. The inductive load on a magnetic ballast can be as much as 20 times the running load. For example, you may think that a 400 watt ballast would be fine on a timer rated at 3000 watts (3kw). However, the inductive load of the 400 watt may be up to 8000 watts (8kw). This can cause the contacts on timers to fuse together, locking your light in a permanent on or off state, or in some cases, setting the timer on fire.