"What can we do to signal a definite switch to "Game time is now!"?" Yamikuronue.
So many of the other responders have pointed out excellent behavioral techniques to establish a routine that encourages a specific action. Visual cues and Aural cues are the big pair that establish the beginnings of routines. I can only suggest that you hand out rewards to establish the connection between a stimulus ( lights low ) and an action ( serious gaming! ) to create the desired behavior. XP or a sweet situational bonus is always nice.
Other stimuli that you can use to establish a cue for a shift in time could be to have the players recite a very specific oath which should require the players to say something their characters would say. For instance, saying Green Lantern's oath before playing a Lanterns game would be incredibly appropriate, almost like making your own opening theme song. This encourages thinking, and thinking is very useful for getting into character.
Another cue could be tactile, but should be unusual and have no other associations, like buying fake noses or other costume pieces. This is very similar to the "Golden speaking [Item]" rule, where normally only the person who holds it can speak this version would have a rule more like "While I am holding this, I can only roleplay my character - Until I give it back to the ST,". To encourage this rule, the item being held out of game should grant a bonus in game. Funny hats, costume jewlery rings, rolling your dice into a special tray or bowl would all be aids for this particular stimulus ( and also give you something to laugh about if you're imaginitive and sadistic about it). This is about as useful as a "reminder-ring" if you or any of your players use a similar device.
If you have time to set up, another valuable stimulus could be smell or taste if you're into candles or particular aromatic dishes. I recommend the former over the latter, as it may encourage eating over roleplaying; however, food is a very strong reward. (Can YOU imagine a situation where "Cookies for Roleplaying" would not work? I think not!)
Finally, you can always just reward players more for roleplaying longer. This may alter the experience curve you may or may not have intended for the characters, but this is the single reward that any ST can grant. It does not cost real-world resources, and only increases in value as the individual character's power and thusly advancement costs increase. This is a cue that is only delivered AFTER the behavior and may not be as strongly linked to the action you want. If you explain to the players that you will get more XP for roleplaying longer and STICK TO IT, it should work well.