I struggled with SI for 4 years and have been 4 years "clean" now. I have heard people say that holding an ice cube in their hand helps, as it's painful but does not injure you. Punching pillows and journaling help me, as well as playing the piano. Sometimes I just write down whatever is causing me to cut and rip it in to tiny shreds, just like I want to do to my skin. You can feel the cutting and ripping action...it's just directed towards the real cause. Sometimes I think it's best to distract yourself, maybe with a friend or a social outing or a funny TV show, because moods can change. Sometimes it's best just to go to bed if it's late enough, because things sometimes seem better in the morning (at least for me). I also sometimes drew on my arm with red pen as if I were cutting, but I think that one might be dangerous. It's good to talk to others sometimes too if you have that luxury.
There is also a game called the butterfly game. You draw on your arm, in Sharpie or pen, a butterfly, then put the name of someone you love by it. If you injure yourself, you kill the butterfly. If the butterfly fades naturally before you self injure, it escapes and lives. Your mental state affects the ones around you, so this helps you think about them.
Best of luck, man. Stay strong.