I was cutting since I was young and for a long time I didn't really understand why, but I eventually figured out that a lot of it was a way of 'punishing myself'. My emotions would build, thoughts would ruminate "You're bad/ugly/a P.O.S./worthless/disgusting/stupid/etc" and then "You need to 'be punished' because 'you are all of those awful things'". "You don't deserve to live without 'punishing' yourself- You MUST 'pay the price". Somehow 'punishing myself' by cutting or self-harming in just about any way possible, seemed to relieve all of the tension and self-loathing/intense emotions that were built up inside me. I quickly became addicted to it. I didn't even have to think twice about/couldn't think twice about it because I'd already be slicing away at my arm or whatever was convenient in the moment. (Cutting with anything that would cut/cause me to bleed, cutting just about anywhere- including my face, punching anything that would cause my knuckles to bruise/swell/bleed, hitting my body & face to the point I could see bruises, throwing & breaking things, etc) DESTRUCTIVE! That's a good word for it.
I absolutely relied on it to 'get me through' whatever was happening at the time. The compulsion was always bigger/stronger than me. I am one of those people that likes to see the blood. Not sure why? Perhaps it felt like 'proof that I did something RIGHT' or that I felt I had control of something? It's like "Good job! Fabulous bruises, nice deep cuts, blood... Ok, we're good." check. check. and check.
Although I don't self-harm as much as I used to (I'm definitely not 100% over it), I still have a fascination with sharp objects- almost like getting a high just from seeing them because I know the power they hold. Sound strange, or does someone understand where I'm coming from? Even when I was trying to avoid cutting, I HAD to have a knife (or something) somewhere, anywhere, that would be easy to get to 'just in case'. I've felt a certain safety with knowing I have my "TREASURES", but it's not in case I have to defend myself against someone... It's in case I need to 'attack' myself/ defend OTHERS from ME. (That whole punishing thing again).
I was going to suggest the rubber band technique that was mentioned above- I can't see getting enough relief from it myself- but it might work for others. I've seen a technique of using a red marker (washable) to draw on your body- as though cutting- in order to actually SEE your actions and be able to FEEL the coldness of the marker on your skin. That technique could look pretty scary if you went out in public w/o washing it off though!
Here's another: Taking an old rag or towel and ripping it into as small of pieces as possible- ripping paper might work, but I think the towel would give more tension-release. Maybe buying some cheap thrift store glasses, dishes, or anything breakable and finding a a good place to throw those ******! 
Idk how 'healthy' these options are in other people's eyes, but it might be better than SELF-harm depending on who you ask.
For me, I try to fill my head with ruminating thoughts of "Don't do it. You can go without this. You'll be ok- You've survived before without giving into it." Distracting myself works sometimes but can be extremely hard. A lot of times (especially when it's more rage-associated feelings) I liked to take a walk. "Thankfully" I had a friend that was also a cutter, so we could 'walk & talk'. We were fortunate in that we were both willing to walk at pretty much a moment's notice, were both insomniacs and especially loved walking at night, and walking fast- all that helped curb our built-up emotions. Fighting/Versus Fighting video games have been a great source of comfort (Soul Calibur!) I think yesterday when I felt like cutting I just distracted myself by getting on the computer. While hospitalized I learned that putting puzzles together and coloring were actually a good distraction. I know sometimes these suggestions can tend to feel 'not enough' when you're having those urges but I figure it can't hurt to suggest some ideas.
When my ex-bf was in drug counseling (yep, that's right) a project they gave the group was to fill a box of stuff with things they could do/use that would be convenient if they ever got the temptation/urge/craving to 'use'. I imagine the same concept might work for self-harm too, because it's all about knowing your options fairly quickly and having them readily available. It's easy to give in rather than think of options in the moment and often difficult to even see that you have other options when you're feeling that way.
Does anyone else think of their scars or anything as 'battle wounds'? For me a guess I like seeing the scars because they're proof of my struggle & fight to... exist. Self-harm is like an old friend that I can't seem to/don't want to get rid of. I understand it's not healthy in the traditional sense, but it is what has helped me get through one more moment and one more day... -Jamous-