Experimentation is always the rule. Unless the room is very live acoustically you can easily overdo it, so proceed judiciously. Begin with a large thick floor rug centered around the listening area; may not want to cover the entire floor. Wood floors are beautiful & too much carpeting may make the room too dead anyway. Hook rugs / thick tapestry / deco blankets / your foam posters hung at strategic locations (walls directly behind speakers, first relections points etc.) work well. Diffusers or absorbtion on ceiling at reflection points are helpful (I sprayed my entire ceiling with 1" thick cellulose insulation fiber - the bleached white variety - works fantastic acoustically). Slap echo from the wall behind the listening position should be approached a bit more carefully, because too much there (as in a thick blanket) may absorb excessively. Experiment with thinner & smaller materials if too much absorbtion is noted. That compressed foam or tapestry might be ideal. More sophisticated devices include Argent Room Lenz, ASC Tube Traps, RPG diffusers, corner & triangular pillows, Room Tunes.
If you really want to do this right, consult with member Rives regarding professional approaches.