How to steady a camera with long zoom?

So i'm going to a concert and i bought those high zoom lenses for my iphone 5s. Its 17x zoom and i love it. It works great but what happens is tht its so hard to steady some pictures come out bad. Is there a way i can steady it?

Added (1). ok i have a tripod too but that doesn't help when you're standing. I'm going to concert so ill probably be standing and even if i'm not there won't be a place to set the tripod

They do actually make tripods and tripod attachments for iphones.

There's no magic way to get rid of shutter shake. The longer the zoom the faster you need the shutter to be. The rule of thumb is you shutter speed can be no more than 1/focal length to avoid shake. Apple says their camera is base 35mm, and the math doesn't work like you think, so I would imagine a 17x zoom ends up at somewhere around 200-400mm so you need a very fast shutter speed to make that work. Concerts are hard as balls to get good lighting so fast shutter speeds are hard to come by.

Best you can do is rest your arm on something, or better yet rest the camera on something like a railing or top of a seat if you have one.

When I'm shooting with a long lens (300 mm of longer) or using a zoom lens like a 200-400 mm lens, I use a monopod and keep my shutter speed at 1/500th second or faster

Since you are using a mobile with an attached lens, you will have to buy a special monopod attachment for your camera. I'm sure you can find them on Amazon if you look

As a rule of thumb, you should use a shutter speed that equals or exceeds the focal length setting. Thus if your zoom is set to 500mm then shoot at 1/500 of a second or faster. If set to 800mm then 1/800 is the shutter speed of choice. OK to shoot at a slower speed if you can keep the camera steady. You can practice by taping a mirror to front of the lens. Set a flashlight on the mantle aimed at the camera. Adjust your position and the flashlight so that the mirror reflects a circular bright spot on a wall. Practice pressing the shutter release while watching the spot of bright light. This training method allows you to see your unsteadiness. Now practice bracing the camera with both hands. Practice breath control, you take a deep breath and exhale and shoot as the last whiff of air passes your lips. This is how a target shooter practices. All this and no caffeine but know that a tripod is best. Next best is to brace the camera against some support like a column or door frame. Practice is the key to successful camera handholding.