I agree that meditation can help, although I found that for me the main difficulty to
learn meditation was to make the inner voice to shut up. See the problem here?
There are another ways, though.
First of all, it is your head and your inner voice. You can control it. Make it louder, make more negative thoughts. Don't worry, not all of it is going to manifest.
Ask your mind - can you throw any more negative thoughts at me? Tease it! Tell him to bring it on! Eventually (quite soon, actually) it will overload and won't be able to come up with anything new. Then you can start to introduce positive thoughts.
Second approach. Speak out everything that you inner voice tells you. You should be alone for this. Speaking slows down the inner dialogue. You can also read a book, a poem by heart, sing a song. You can even keep quite and make a "tongue dance" - twist and shake your tongue inside your mouth. It is very convenient when you are at a public place - any tongue movements suppress the inner dialog.
Play with it. Make it louder. It is usually easier then to make it quite. Try to imagine the remote control that makes a sound louder or quieter. Locate the sound. Where is it? Is it in your head, near you right ear, near your left ear? Above your head, below? After you locate the sound, move it. A little bit at first, then launch it to the moon. You can always bring it back if you want.
What is the pitch of that voice? Try to change it. Make it thin like when the tape is fastforwarded. Or slllllooooooowwwww, like in a player with near-dead batteries. Make this voice sexy or make it have a sore throat, coughing and sneezing.
Another trick is to look at some object and start listing the differences between this object and you. Do it aloud.
Say for example -
This kettle is small - this is not me.
This kettle is yellow - this is not me.
This kettle is empty this is not me,
This kettle has writing on it this is not me.
Go on until you suddenly won't have anything to say. At this moment your inner voice will be silent and if you look around you might notice that your visual perception has changed slightly. At first the inner voice will come back very soon. If this happens, continue the exercise. After some practice the inner voice will be able to shut up for longer period of time.
And the last one is pretty rude. Every time you hear something you don't like in your head, just scream inside your mind
"Shut the $%^& up! Shut the $%^% up! Shut the @$%! up!" It works sometimes