Well, this is all assuming your not trying to make one spiffy spot for the Terrain Showcase and its a fairly-sized map.
Some of this is probably no-brainers but whatever, here it is:
VisualizingDoodads can be a double-edged sword. They can either help make Terrain look better or they can make it appear unrealistic. First, you should ask yourself what you are trying to portray. You can do that whole "close your eyes and visualize" thing too, whatever floats your boat,

but it's not necessary, just try and get an aim for what you are trying to portray
Placing the DoodadsPlace doodads for a purpose, not just randomly and for no reason (there is exceptions to this, depending on the type of map you are making). Compare your doodads to the area around it, and possibly even the map as a whole. Does it fit in? Is it helping in portraying what you visualized? You don't have to do this for every doodad, just what you consider to be most important, otherwise you'd eventually go crazy all day figuring out where to place that certain tree

. Also, don't feel like you MUST place a doodad on every spot of the map, like I said earlier, they're a double-edged sword and can be more degrading than helpful to a map if not used properly. If your not trying to go mega detail, with just some doodads here and there, its probably best to set doodads not too near nor far apart.
The best way to learn is by seeing what others have done in the past. Now, I'm not saying copy their whole map, but look at where they set their doodads and try to figure out what the purpose may have been in setting it. If you make a terrain similar to theirs, try and plug it in. Experimenting with doodads is also invaluable in learning how to 'illuminate' your map. Don't expect to be a master Terrain/Doodad Placer in one night, (psh, I'm nowhere near being one

) it takes time.
Well, I hope this ranting was helpful in answering some of your questions, happy mapmaking!
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