The most frequent questions I have is how to damp speaker cabinets and it's not easy to give general recommendations. Below is seen the materials I use:
1. 8-10 mm felt material
2. 30 mm polyester foam, acoustilux
3. Bitumen pads (found in car-fix shops) (Harald Nyborg in Denmark, the cheapest place to find this excellent material). Also available from Jantzen Audio in 4 mm thickness. Always optional when buying kits.
4. Pure sheeps' wool material (not easy to find; found mine in a shop dealing with knitting stuff). Also available from Jantzen Audio.
5. Egg crate foam, can usually be bought from all shops dealing with speaker parts. Also available from Jantzen Audio.
That's about it.
Download Jantzen Audio catalog on damping materials here.

NB:
Based on numerous mails, it appears a lot people think bitumen pads will "dampen sound waves", i.e. absorb sound. Bitumen pads are actually excellent in reflecting sound, thus do not absorb sound at all.
Bitumen pads adds weight to the cabinet panels and reduce panel vibration - and that's a different story.
Felt and foam will absorb sound and soundwaves travelling through felt/foam will reduce speed and energy will be converted into heat (- don't worry, not a lot of heat!)

FELT MATERIAL AND POLYESTER DAMPING SHEETS

Above my preferred damping materials: 8-10 mm felt and 30 mm polyester damping sheets (acoustilux).
Both available from Jantzen Audio.
To fix the felt sheets to interior panels I use vinyl glue. For the 30 mm polyester I use staples - or nothing depending on actual application.

Egg crate foam in various qualities. I suggest 30 mm egg crate foam as part of the damping material for very large bass cabs.

BITUMEN PADS

For Danes: https://autofix.nu/produkter/8130-vibrationsdaempende/58777-lydisolering-resonansdaempning-sort---selvklaebende-maal-500x500x4-mm---antal-1-stk/

Jantzen Audio has an extensive range of 2-4 mm bitumen pads. Download data file here.
Despite being self-adhesive, I strongly urge using glue with any bitumen pads. I use a thick layer of glue used for vinyle tiles. Check QUATTRO file to view how.
Also check "tips" file here.
I've had self-adhesive bitumen pads falling off after a couple of years due to not being glued.

Example of damping of floorstander

Seen from side.

Above an example of a common floorstander cabinet damping strategy. 
1. The bitumen pads are optional and if only you want to do a little better, or you may have used light-weight panels needing additional damping.
2. On top of this I recommend adding 8-10 mm good felt material as shown above. The front panel doesn't necessarily needs any damping. Three sides are usually enough.
3. 30 mm acoustilux: Right behind midbass drivers I always add 1-2 layes of acoustilux to reduce early reflections towards the thin driver cone(s). 
4. Quite often we see problems with minor impedance peaks in the 100-200 Hz region and this usually caused by standing waves between top and bottom. Having an internal height of e.g. 1 meter we may have a resonance at 34000/100/2 = ~170 Hz (half wavelength), thus 10-15 cm acoustilux at top and bottom usually cures the problem.