To beat other runners of equal ability, you need to run smarter than them. To beat the clock, you simply need to run the fastest time possible, which is almost always accomplished with even pacing.
So, first decide what your objective is for a given race, then select the tactics to meet that objective. Finally, during training, visualize yourself successfully using those tactics to achieve your objective.
Running an even pace is the most effective way to run the fastest time, because it utilizes oxygen most economically and keeps lactate accumulation to a minimum. Most distance world records are set by running even pace.
Consider the 10,000m world record (since broken) of 26:31.32 that Haile Gebrselassie set on July 4, 1997: His two 5,000m halves took 13:16.74 and 13:14.58; his kilometer splits varied by less than 4 seconds (2:36.7 for the fastest, 2:40.6 for the slowest). Any way you parse it, Gebrselassie's run was a model of economical racing.
Even pacing requires that you know the correct pace to select, that you have good pace judgment and that you have the discipline to hold back in the early stages of the race, when the pace feels easy.
The best time to learn race pace is during training. When you learn what it feels like to run at your goal pace, it becomes natural for you to maintain that pace during the race. Do some training at your goal pace for important races, so that you can more easily settle into that pace and so that it becomes natural for you to maintain that pace.
Running even splits is also often a good way to beat other runners. By running an even pace, you'll gradually catch runners who started the race too fast. Catching and passing other runners during the race will give you a psychological lift, and will weigh heavily on the other runners.
Of course, you don't know whether the other runners will come back to you; you may let someone go in a race and never catch him. If you run evenly, however, you'll have the best chance of beating someone who has started too fast.
Running an even pace is also your best tactic for beating someone who starts off slowly and tries to pick up the pace during the race. If you run evenly, the other runner won't get the psychological reinforcement of making up ground on you quickly. The other runner will also be using energy less economically, so if she does catch you, you'll be able to hold her off.
In some situations, it's necessary to abandon the even-pacing rule and start the race somewhat faster than the pace that you can maintain for the entire distance. For example, if it's important to you to beat a specific runner, and that person starts off quickly, then you may want to run a bit more aggressively so the other runner doesn't build too big a lead. Chances are that the other runner will pay a high price for the fast early pace, and you'll catch her later in the race.
Environmental conditions can also affect your racing tactics. On a windy day, you're best off running with other people. This may require running slightly faster or slower than you otherwise would to stay in a group of runners. Try to let the others do most of the work blocking the wind for you. If the pace is slowing, you may want to suggest to the other runners that you take turns leading into the wind. (Of course, the other runners may tell you where to go). This will help the group overall, and may help you all catch and pass other runners.
Similarly, on a hot day, you need to reconsider your pace. Jack Fultz won the 1976 Boston Marathon in 90-degree heat by wisely choosing a conservative pace early in the race, then steadily passing other runners as they wilted in the heat.
If you're catching someone in a road race and want to pass him, do so gradually, and run just behind him until you're ready to make a decisive move. Then run by strongly and keep your momentum until you've built at least a 30-yard lead. At this point, he'll lose contact with you mentally.
Never look back to see where someone else is. This gives the runners behind you a psychological boost by letting them know that you're tired and are worried about them. (You may very well be, but they don't have to know it.)