Make the track easy to read
Use a bright block for the track bed and a different one for the rails, like red with white or blue with white. Draw the track as a three-wide strip. A contrasting color makes every hill and turn easy to see from far away.
Use real rails if you want to ride it
A rail needs a solid block under it. Lay rails on a path of blocks, and a rail placed on a block one step higher tilts into a slope on its own. You can only turn 90 degrees with plain rails, so build wide, smooth corners.
Powered rails are your lift hill
A powered rail gives a minecart a push when it has a redstone signal, such as a redstone block or torch next to it. They are made from gold ingots, a stick and redstone. Use them often going uphill, about every two or three blocks, and now and then on flat track.
Real loops are not possible, so fake them
Rails cannot go upside down or straight up, so a minecart cannot do a loop. Build the loop as a decoration beside the track, or run the cart through a tunnel that looks like one. A tall ring of colored blocks still looks great.
Hold up your hills with supports
Real coasters stand on columns, and yours should too. Put a column of fence posts, logs or gray concrete under the track every two or three blocks, and add a cross beam on tall hills. Taller hills need more supports.
Add a station and a train for the fun parts
A station with a striped roof, a platform and a fence makes the build feel real. Make a short train from three or four blocks of colored concrete in a row. Put a few lanterns on the roof so riders can find it at night.