Placement

 

How to position speakers.

Give them some air, against the wall or in a corner is not a good idea.
I use solo piano recordings to position them. If the arms of the pianist are too short, they are too wide apart.

A more useful advice can be found here: Audio physics.

Genelec offers a nice explanation of the bass response in relation to the distance between the speaker and the rear wall. When positioned wrong the bass radiating of the wall cancels out the bass radiating directly from the speaker.

Measuring

A more advanced method is to measure the system response using a calibrated microphone.

Coupling/decoupling a loudspeaker

You can put a loudspeaker on the floor, use spikes, a viscoelastic material or stands.

Some thoughts on  this subject by Gary L Koh.

References

1. Setting up Loudspeakers in a Room - Audio Physic
2. Understanding Coupling/Decoupling - with particular reference to loudspeakers - Gary L Koh
3. I am not getting enough bass? - Genelec