Come up with a basic concept and feeling for the interior: what atmosphere are you after? What does the combination of local circumstances, the current inhabitant, and the purpose of the interior tell you? What story do you want the interior to tell?
Try to create small implied scenes within the space of the interior to give the player a feeling of it being a living space, but don't clutter it without purpose.
Alternate between looking at the interior in the editor with looking at the interior ingame. Walk through it like a tour starting at the door through which you enter, and keep thinking "what is missing here, and where is it missing?". Then take the tour out to the entry again.
You'll get further with good design than with an abundance of blunt effort: knowing what not to add is as important as knowing what to add. Also remember that this is a game: a perfectly realistic setup of items, furniture, utilities etc. is not what you're after. Work with representation and suggestion. Read up on the world the interior is supposed to be a part of, and make it feel like it really belongs.
If you have a specific idea about how you want things to look, but can't find the right pieces, be creative. Try out different methods of acheiving what you're after. Don't give up.
Keep the prospective player in mind, but always use your own ideas and visions as your main guideline. Don't give up.
Don't give up.