Big untreated room with a fluffy pillow right infront of the amp (laying down, without being intrusive to the mic stand) and a bit of foam/ pillows at the sides of the amp- "standing up" (leaning against the amp if need be).
If you have enough shit left over, even have something dead behind the mic stand- rapists and murderers work well (;p), or if you dont have anything left over, just "aim" the amp towards a corner, but from roughly the centre of the room.
Just use your logic when placing the amp as to where the reflections are going to go, and where you want them to go- together with a few pillows will work fine ina big room.
Small, untreated rooms such major ass.