A good assistant restaurant manager is ideally one who is more than just a favored employee who makes 50 cents more an hour and holds the store keys. He is someone who makes the general manager's job easier, all the while prepping himself for one day getting a store of his own. It's an occupation that carries much responsibility. The good news is that it is just as rewarding as it is stressful, and by employing a few basic methods, you can parlay being a good assistant into one day managing your own restaurant.
Learn your role. There are several types of management roles that assistants fulfill. For example, some higher end restaurants are extremely specialized; there is a kitchen manager, a front of the house manager and an assistant general manager. Other times, it's more talent based, as in one assistant takes care of the paperwork, the other handles the hiring, and so on. Either way, familiarize yourself with every detail of what is expected of you.
Show up prepared every day. It's vital to be proactive and anticipate every possible twist and turn of each shift. A good assistant manager never goes on autopilot; he has a contingency for anything that could go wrong in a shift. For example, if the truck is running late and won't arrive until the lunch rush, consult with the shift leaders in the kitchen and front of the house and inform them that they'll have to step it up for a short spell. Leave nothing to chance.
Communicate regularly with both the employees and the general manager. Not only is this good in giving the latter a feel for the state of the restaurant, but it also works well for handling staff problems before they become too big. A good restaurant assistant will handle any potentially incendiary issues before they reach the GM's desk.
Train each employee completely and use the same methods for everyone. It's vital to have a set program in place as opposed to instructing each new hire on the fly and hoping for the best. Also, follow up on the training. The worst thing that can happen is for a culture of "this is how we're supposed to do it vs. this is how we really do it" to pervade.
Delegate responsibility. One common mistake of new managers is that they try to do everything at once, leaving them ineffective when it comes to actual management. For example, it's fine if you have to help run food from time to time. However, when you spend an entire hour helping out a struggling kitchen instead of working the customers and overseeing things, you're not being the most effective manager.