A couple easy things first: Go to the components window and use the menu to "purge unused". Also, go to the materials window and do the same. This can go a long way to cleaning up the mess that can be made during the design process.
Also, reusing components as much as possible can help this. Four legs, for example, can almost always be identical copies of a single leg component. Using rotation and the scale tool, you can get mirror and inverse images of the joinery in that part (if you draw the joinery at all).
There are some other extreme measures you can do but can make generating a cutlist a little trickier. This involves only drawing half or quarters of a symmetrical part and then copying them, mirror and hide some lines. This gives you the look at a small file size but some of the cutlist generators get confused by this technique.
Another one is round surfaces. Anything that's curved is really made up of facets. These facets are individual bits of geometry and so the more facets you have the bigger it is. You can adjust the facets of arcs and circles in the Entity Info window.