Think in three dimensions. Mount furniture on the walls and/or hang it from the ceiling. If your apartment has high ceilings, you might consider creating a loft bedroom
Use furniture that has more than one function. For example coffee tables with built in storage space, sofa beds, coffee tables that can transform into full height dining tables, poufs and stools you also can use as small tables and so on
Adding wheels to furniture makes cleaning and rearranging much easier
Regularly sort through your stuff and get rid off things you don’t use. Recycle, sell or throw away.
Use small size furniture
Go for closets and bookcases that are high and narrow. That way you get maximum storage while saving floor space.
Drop the huge loudspeakers and hifi system. Go for the wirefree mini speakers.
A large wall-mounted mirror makes a room seem bigger.