If you haven't seen the exterior portion of this two part series check it out here.
There is also a world download, and a link to the Flows HD texture pack in that Instructable.
These tips are not just for "modern" houses! Most of them will work for all houses!
Step 1: Wall Patterns
I like to add some wall patterns to my modern houses. like the one in the picture. I especially like to put them around doors.
Step 2: Lighting
I prefer sea lanterns in my modern houses, especially in Flows HD just because of the metal bars, and I think the blue color looks great with the white.
Step 3: Stairs
I would highly suggest using slab stairs, as it takes away the "normal" look of most builds. I also like to add some pillars to make it look more realistic. Honestly how many floating stair cases have you seen? Besides in Harry Potter?
Step 4: Windows
I like blue or light blue tinted windows; like I said with the lighting, I like the blue against the white.
Step 5: Home Entertainment Center
This is an important step in any good house. Don't just put a painting on a wall and call it a TV. Make it look like one.
Step 6: Gardens
I like to add some form of flower gardens in my houses. These are my favorites.
Step 7: Games
I like to include at least one of three games:
A pool table
A ping pong table
These designs are pretty simple, but if you can't figure them out, tell me in the comments and I'll see what i can do.
Step 8: Other Decorations
I also like to include a desk of some kind, whether it be a PC, or a book desk, I usually have one.
I also like to "mount a moose" and put it on my wall.