There's a couple of things I always do when remounting a tube/tire. After inflating the tube before insertion inside the half mounted tire, I always take some talcum powder and put some in my hand and run the tube through it a couple of times to coat in with the powder. It makes the tube a bit slippery when inside the tire and easier to get the valve stem lined up with the rim hole. If you want, you can even get some powder inside the tire. It won't hurt a thing. Then as you work the second half of the tire over the rim, you must keep the tire centered and force it down as deep as possible into the rim. This gives you just enough more tire to leverage over the rim as you work it around the wheel. Always remember to give your rim strips a going over before remounting a tire to make sure the spokes are covered.