use a mirror and a friend. sit in your mix position and get the friend to move the mirror around, anywhere you can see your speakers stick a shit load of foam. These locations will be on the ceiling one on the left wall, one on the right wall, behind the speakers in the middle and behind your head in the middle.

You may find you get better results by having a bookcase or a diffusor directly behind you as that will scatter the sound rather than absorb it potentially resulting in a more natural sound. At least that's how most of the studios i've seen are configured.

I've not done much a/b testing to confirm all this for myself but that's what I'm led to believe is the best minimal solution.

As Z says ... bear in mind that generic foam may have a weird frequency response so you may not be getting the same quality solution you'd get with something like acoustic foam from studiospares or auralex.

But auralex form is about 50 quid a panel so you'll be quids in anyway :)