I'd say the easiest way to have good coasts is to use your favorite image editor (not paint) that supports the border effect, which is rather simple and should be included in the gimp, for instance. Just make your terrain in a 8-bit black and white 512x512 bmp with a black background and no alpha channel (bmp should take care of that). White is the highest possible, black is the lowest. Put your land in the middle of the image for best results, haha. If you have islands, connect them with a bridge of 1s to prevent errors. If you're wondering how many tiles you are taking up, go into your grid options in your graphics editor and set them to 32x32. If you have lots of really low terrain and you can't tell it apart from black, use your selector tool on just absolute black to see where the borders are. I haven't found a way to make it automatically count the tiles, but you can get a rough idea using this.
When you're done, you just need to border your way down to 0, so it's easiest to blur your map edges into the black (or not, it's at your discretion), go back a bit on the blur using the fuzzy select tool on the blackness with an option of 9 and convert to all black. Afterward, invert your selection and then you need to border your land (with 2 thickness to prevent mess-ups) with first 9, then 5, 3, 2, and (maybe? the post above says it is unnecessary) 1. Tada, no need to click and edit eeeevery single tile. Just use lndromat to turn it into a land map after exporting the file in the .raw format. If your file size isn't 262144 bytes, you did something wrong (probably an alpha channel or not 8-bit).
I find it fun to use a fractal creator, a free one can be obtained for the gimp. Make a suitable fractal then use THAT as a landscape, it's pretty awesome.
For lndromat, use all the default files except for the .raw heightmap you made. Next, you need to put in your terrain textures. You can only add one terrain type when you do this process. To be able to get all the terrains you want, you have to follow a quick process. First, I'd suggest you preview your terrain in surveyor to make sure it looks like what you want.
Use eden project to setup all your textures and such. Yes, I know, eden project is evil. However, there is a way to fix the errors it places in your map!
You may need to open the map in surveyor after using eden project to insert your textures before the next step, then just save and close. I'm not sure if that is necessary to put the broken but "correct" low res textures in the map to overwrite the default texture put in by lndromat, it's what I did, though. At the very least, you can do this while you are previewing the map with its correct countries. Experiment and find out, preferably telling everyone here if it worked or not, haha.
Finally, simply use the lnd tools, made by the same guy who made lndromat. Just run lndsertx! It will put up two prompts, the first lets you pick the map you want to fix. The next picks the output, but you can just pick the same map and overwrite it with the fixed version. Afterward, all you gotta do is type 4 and hit enter, and bam, it's all fixed! Yep, that's right, this can fix everything.
I used this process to create a totally original, 244 tile map based on a fractal. It works! The only flaw is that the coast tiles show up as solid 2 terrain land until you zoom in, then it's better forever. That's incredibly minor, IMO.
I also created a random city generator (people with corresponding houses, buildings, everything.. all according to specifications, such as pop; if it has a creche, etc; size; blah blah... it even prevents building collisions! no manual crap, just click a button and it's in your clipboard. i love abusing parenthesis), an automatic tree creator that imports your .raw heightmap to make the trees at specific altitudes and bases their type off of the nearby trees, and a random rock creator that also uses an altitude range based off the heightmap. They're really sloppy right now, though.. I might fix them up and upload them if there's any interest.
Of course, I used worldmod to find coordinates and touch up and everything. If you can't get worldmod to work, rename "readme1.txt" in its install directory to "readme.txt". That should fix it.
If you do all this stuff, making a map is no longer torture.