Having something else to focus on can help - chant a mantra (just "om" works okay if you don't wanna look for anything more Perfect For You), count your breathing and reset the count every time your inner voice interjects something or when you get to ten, stare at something (I like just staring at the grass sometimes and watching how my visual centers start acting when they get really bored with looking at it).
A big key is to stop being angry at yourself for failing to stop that internal monologue. It's a hard thing to do! Instead of beating yourself up, just shrug, forgive yourself for doing it (maybe even say "I forgive myself for failing to quiet my inner voice" out loud, this activates different parts of your brain), and go back to whatever you're doing to focus on next-to-nothing.
If you can block out the time, I found that my meditation skills got a lot better when I stopped trying to do it for a few minutes at a time and started setting a timer for thirty minutes. Some days I can stop thinking easily. Other times I keep on fighting it for like twenty minutes. But doing it for a long stretch like that gives you more chances for your twitchy brain to get bored and STFU.