I will add a few more tidbits of info on how to block then. If you are near the edge of a panel, let the sandpaper be a bit over the edge so that you do not dig a trench, and the panel flows straight to the edge. If the block can hit two edges in it's length, like the B pillar, go ahead and overlap both. Also, whenever possible, sand diagonally at a 45 degree angle, so that the length of the block covers the most possible area. If you do a large area such as a quarter panel top to bottom with filler, don't just sand top down, or bottom up. It may block straight according to the guide coat, and if you feel it going from the front of the car to the back, but it may not be straight top to bottom. In a case like this where you are making a crown, you should turn the block so that it is lengthwise top to bottom, and hold the edges then press down on them. Sand this way so that you can take any lumpiness out that may have formed while roughing in the filler. My driver's side quarter panel is Bondo from top to bottom from the back of the wheel arch flare to the tail light due to someone's careless driving, so I learned this while doing that. The hardest thing to form in that case was the body line creases on the top and bottom of the panel. I did it with a air file using it's long, straight shoe to make straight lines. It also allowed me to fine tune the shape of the step, as it is not a simple turn, but a gradual one over a 1cm wide area. To mark out where the line should be in filler after initial rough in, use masking tape by sticking one end at the start point, and then stick it at the very end where it should be. Now, you can push the rest down. This technique of "snapping" a tape line (like using a chalk line in carpentry) sets a straight line that you can sand to. If you stick the tape as you roll it out, the line will not come out straight. You can also use things like wheel arch trims to figure out body lines by putting them on, then tracing with a pencil so that you have a reference point to sand a set distance from. On a car that is so geometric in shape, these techniques are easy to use. If you start playing with a car with complex curves, you will have to add more techniques to make it work visually.
Now, here's another one I will tell you about. It's a pair of minor "tune-ups" I will combine into one that can really make the body stand out. First off you will need to have the panels aligned as good as possible before starting this. Then, you will need to grind down to bare metal around the door handle/lock cylinder area, as well as around the panel gap on the door and quarter. If you want to see why, look at the area around your door handle and lock cylinder and notice that it is slightly distorted as stamped. Next, put down a skim coat of filler about 6 inches wide from the edge of the door and the quarter. Make sure the area around the handle inset is covered, but that the inset itself is not full of filler. Now, you can cut the door gap back open with a razor blade as it sets up, but before it is fully hardened. Next up, sand the whole thing with an Air File until it is level, and you can almost see through the filler in places. Follow it up by sanding the edges smooth with a slightly rounded edge using 220 grit, and blend the filler into the jamb. The door will now look really tight and well gapped to the quarter and the reflection will transfer smoothly from one panel to the other. I did not do the gap on my car, but I did do the handle area to eliminate the factory pressing flaw in this area. Details like this make or break dark paint colors. This is why you can see 2 cars at a show, both without dents, but one's body just looks better somehow. It's the minor details, and keeping waves out of the surface that makes all the difference.