Structure a.) When placing bridges make sure they are accessible on both sides
b.) Most times indestructible bridges are the best way to go as constantly respawning bridges are atrocious.
c.) When there are too many bridges on a big map don't hesitate to use land bridges instead and turn a couple rivers into reservoirs.
d.) Don't use the same buildings too many times on a map or if you do at least don't put them next to each other (silos and hangers the exception).
e.) Try to use structures that are of the same geographic genre for realism so no space shuttles next to a residential house.
f.) Make sure to put buildings, defense towers, and bunkers next to choke points and next to bridges as this will make for encamped battles.
g.) Try to congregate Tiberium spikes or silos around one another for maximization of resources achievement.
h.) Don't put structures on top of other structures because once the bottom one is destroyed the top one will be floating in mid air.
i.) Put misc. structures like fences and zone walls around Tiberium spikes and defense towers but make sure to leave spots for engineers to occupy.
j.) Try not to put Tiberium trees in non-red zone maps as this will look out of place.
Water a.) If you have the 'bay' file replace the 'ocean' file with it to make less waves for a better looking lake.
b.) Try using the lake tool for rivers instead of the river tool as this will enable you to form and sculpt the river exactly to your specifications with no problems.
c.) Don't surround a base completely with water unless the bridge leading out of it is indestructible.
d.) When trying to make waterfalls use the river tool and make small river strips with a pyramid type shape to give the appearance of the water falling.
e.) Also when making water falls using a lighter color water often makes it look more realistic.
f.) When making lakes don't hesitate to also make several small ponds to compliment them.
g.) Smoothing the terrain on the water border often makes for a better shore line.
h.) As stated earlier putting any body of water by a mountain or ridge enhances that elevated terrains characteristics.
Misc. a.) Don't make too many scripts in a map as this will make the game play predictable after about two attempts at the level.
b.) Sounds are good but too many of them or rash specifications might prove a liability for the map and game play.
c.) Often time the simple zone music is enough for good sound on a map.
d.) Putting Tiberium crystals in the brown center of Tiberium fields gives added effect.
e.) More coming soon.