Black sheets and material goes a long way. use it as a table cloth to decorate that collapsible kitchen table. also put the material on end tables and nightstands. you can also put candles on the table or nightstands to add decoration. even your favorite little Knick-knacks and collectibles can be used to decorate.you can even drape a black sheet over your couch. My favorite thing to do is to take a black sheet and splash white paint on it. you can also to this to a white sheet with black or red paint. just make sure you splash the acrylic paint on the sheet OUTSIDE to avoid messes.
Another thing to do is get little knick-knacks. such as sculls, fake spiderwebs, black lamps, gargoyle statuettes and etc. these can add a sense of gothiness if you place them right.
If you can, look out at thrift stores and charity shops for cheap couches, chairs or anything else that can be used for cheap. also look out for things you can modify to make more gothy.
I like to visit halloween stores or Walmart the day after Halloween to find things I can use for decorating on sale before they store it away.