The best tactic is to play mind games with your opponent. Say things like, "Just 10 miles to go!" when there are really only 8 miles left. Or, just the reserve of that, say, "Wow, just another quarter mile and we'll be done!" when, in fact, there are about 12 miles left.
Believe me, this sort of thing will totally mess with your opponent's mind.
Also, when you are feeling bad, like really terrible and you want to do anything other than continue running, just shout out, "I FEEL LIKE A MILLION BUCKS! THIS IS SOOOOOO EASY!" Not only will that fool your worthy opponent, but you might just fool yourself as well and get a second wind.
If you aren't much one for talking during a race then there are more subtle ways of intimadation. My favorite non-verbal tactic is to swing my elbow really, really close to an opponent's throat. The arm swing will feel unnatural at first, but like anything else you can improve it by practicing the technique in your daily trainingn runs.
Anyway, you'll know this one is working if your opponent tries to spring away from you. Jokes on them though, because sprinting during a marathon is a good way to go into oxygen debt and have a poor outcome. Once they settle back down into normal pace slowly reel them back in and continue with the elbow swinging.
Now and then you will race against a really stoic, robotic type that will just not respond to your elbow swinging around their throat region at all. Don't say that I advised this, but if you "accidently" make solid contact to a man's trachea during a marathon, I promise you that it will not come down to a sprint finish. If you know what I mean. ;) ;)