Well, when it is already big, there aren't "best ways".
Always keep in mind to use low poly geometry for everything you don't really want to show a close-up of and to use components extensively.
Everything that is in your model more than in one instance, should be a component. Even repetitive elements within components should be nested components.
Keep image sizes low. Even if you want some high-res images, SU will reduce them to a maximum 1024 size on either edge. There's no use to use bigger ones therefore. Use smaller, tilable textures instead (though some may look ugly when too big faces are paited with them).
In reality, it's not even the file size that slows down SU when reaching a limit but the ability of the video card to display movements etc. Therefore use layers to temporarily hide geometry that you don't need while modelling certain details. This can also go extensive.
Turn ioff materials while modelling (use monochrome mode - at least you can see where the back faces are) and also turn off shadows. Displaying them slows SU down considerably.