Delete faces that the player won't be able to see in-game. Since tihs is an RTS you'll need to keep poly counts in mind while you model. Sorry, no normal maps in this engine. Also reduce the numbers of "sections" in primitives you create. If you can't understand a word I am saying then I suggest you follow Proclone's advice first.
I normall make a model and then look to see if any two adjacent edges that are on slightly parallel. if there is not much of an angle between them then weld the middle vertex.
also, try to avoid anything rounded. ie spheres, cylinders and circles. these are poly hogs! dont worry if the model looks blocky. you can always make something look rounded by shading the skin.