What should I use for coping?
Metal (i.e.-steel) coping works best. Plastic can also be used. If you want to, you could even use pool coping. Use one continuous piece of coping on each side instead of several small pieces. It's also sucks to get hung up on a coping seam.

What about measurements?
The surface-supporting 2x4's should be no more than 6 feet in length. 4 feet is usually a good measurement because you can simply cut 8 foot 2x4's in half. They should be spaced no more than 8 inches apart (unless you are building a vert ramp, in which case they can be slightly farther apart at the top). Also, when designing your ramp, make sure you have the correct amount of flat bottom. You don't want your ramp to be too fast or too slow.

What about the foundation?
It's easy if you are building on level ground. Otherwise, you'll have to level the ramp somehow. One way is to build the frame first and then level it by shoring it up with railroad ties, cinder blocks or whatever. Just make sure it is stable. Another, more stable, more time-consuming way is to drive a 6x6 post into the ground wherever your ramp needs support and build the frame right onto these posts. Use a post-hole digger (available for very little cash at your local K-mart) to dig the holes at least a foot deep and then pour cement around the posts. Use a carpenter's level to make sure they are straight up.

What about platforms?
VERY important! Most people build crappy platforms. DON'T! A good, level, stable platform makes a huge difference. It should be at least 4 feet wide. Wider if your ramp is really big.

Ramp width?
8 feet is really too narrow, unless you need to fit the ramp in the confines of your garage or something. 12 feet is usually baseline, for a mini-ramp. 16 feet or wider is pure heaven.

What about transitions?
Take your time when cutting them out. Make them as perfect as possible. A rush job results in a crappy ramp. Most people cut one transition as a template for the rest. This is a good idea. Make your template absolutely perfect and MARK IT. That way, you won't get confused about which transition is the template. Use one template to trace the pattern for every transition. If you use a different transition as a template every time, it'll create a compounding error, making your ramp impossible to surface. Also, make sure that when you attach the transitions to the frame, they line up perfectly or it will be impossible to surface.

How do I position the surface support 2x4's?
Attach them so that the 2 inch side will be against the surface of the ramp. If you put them on so that the 4 inch side touches the surface, it will weaken the ramp over time. By positioning them correctly, they don't bow as much from the weight of people skating the ramp. This will just add to the overall life span of the ramp.

What kind of plywood should I use?
Use the most inexpensive 3/4 inch plywood you can find for the transition sides. When surfacing it, I would suggest using 2 layers of plywood that add up to 3/4 inch in thickness (or more). For example, you could use inexpensive 1/2 inch ply for the first layer and then cover that with a better quality 1/4 inch layer. This will give you a strong, smooth surface.

Should I layer it diagonally?
If you have the money to waste, go for it. The point of layering it is this:  The ramp will skate faster when you are riding with the grain and slower when you are riding across the grain. Diagonal layering may provide the best compromise, but it tends to be a big pain in the butt and a waste of plywood. I suggest putting the surface on so that the grain runs lengthwise on the ramp. You cover more ground going back and forth than you do along the coping, so this should make it nice and fast.

Is Masonite a good surfacing material?
Some people think so. I don't like it. It is fast and smooth, but those are the only benefits. It is also very slippery, which I don't like. Rain destroys Masonite quickly, so it is usually no good for outdoor ramps. It works OK indoors, but creates a lot of dust when you skate it. If you ever skate an indoor Masonite ramp, you'll notice that it gets really dusty and hard to breathe after a while. Getting that crap in your lungs is really bad for you. There is a new substantial wood you can buy, it's not as cheap, but if you use it, it is ten times stronger, and it makes up for both plywood and Masonite. It is called "Polly board." Click here for further information.
NOTICE: When putting a new layer of plywood on the surface (over an existing layer), offset the seams. If the seams on the new layer line up with the seams on the old layer, it'll break up faster. By offsetting the seams, you are avoiding this problem.