Wear a wristband or two. Or wrap up one arm with an ace bandage and say you pulled a muscle in your arm or something. Have a big watch? wear it. Or bring a light jacket or sweatshirt and drape it over your worst arm..if you don't mind carrying it all night. Folded bandanna tied around one of your arms work too. Make up might work a little if the lighting isn't too bright where you're going. Most make up doesn't work well with scars unless it's caked on leaving it to look unnatural and obvious. But it might be worth a try. I've heard Dermablend works wonderful. It's light and covers well, except for raised scars and it doesn't come in very light shades. L'oreal true match mineral foundation is good too. To choose the right color pick one thats closest to your skin color or a little lighter. Sometimes if you spray a tiny bit of hair spray over the make up and then add powder to reduce shine it stays on longer. Fake tan (careful when choosing one so you wont turn orange or anything) can make them less noticable too. L'oreal brand is good for that. Be very careful if you have open cuts though. They can easily get infected with make up and stuff. But if you don't have open cuts and don't mind appearing dirty...you could use dirt in a non obvious way if you're desperate. If you do have open cuts but really need to cover them, use plenty of liquid or spray on band-aid (which can sting bad sometimes just so you know) before applying make up or anything and wash it all off as soon as you can. If all else fails, wear a light long sleeve shirt. If you're worried about sweating a lot, a product called Ceratin Dri can probably help. Its in a small bottle thats has a little roller and it goes on with your deoderant. And if you need something to stay in place (like a bandanna or wristband and are worried about it moving) simple double stick tape will be your hero. Make sure you have some excuses planned out ahead of time just in case someone notices or gets suspicious. And don't over accessorize your arms...it'll look tacky and weird if you do. good luck. =]
I self harm too...sorry my answer is so long.