Floorplan: Especially in a studio apartment, consider valuable floorspace and see if you can't swap out your bed for your desk, or move the bed to a different wall to open up an initial feeling of space when you walk in the apt.
Sightlines: What's the first thing you see when you walk in? Is it a symmetrical art hanging? Is there so much stuff that you don't know where to look first? Is it a made bed? Walking into your place from the pov of a stranger can help shed light on where there's too much or where things might look empty.
Empty Space: Clear that clutter! If you're walking into a mess of a landing strip with nowhere to set your bag/keys/coat down then the space is going to feel small no matter what the rest of it looks like. Is there any empty surface in the place or is there stuff everywhere? Aim to have an empty table or shelf or wall- somewhere for your eye to rest and not be looking at more stuff.
Keep Colors Similar: In a small space you can certainly mix patterns and colors and make it feel rich and layered, but be careful to stay within a chosen palette so that each part of the space is seamless with the next. Visually this keeps from the space being chopped up and feeling small.
Light: If the apartment is bright, it will feel bigger. Remember 3 points of light in each room is optimal, though you can do more. Keep windows clean and try to open up your shades during the brightest part of the day.
Go Vertical: You've heard us say it again and again, but even something as simple as hanging artwork instead of having it leaned up on top of a dresser can bring the eye up and make a space feel expansive!
How do you optimize your small space?