I use Kydex knife sheaths 85-95% of the time.  I don't often attach anything to the Kydex sheath.
If I feel like I should attach stuff to my knife sheath I strap a MOLLE utility pouch to the Kydex sheath.  I do this by running paracord through the rivet holes horizontally across the front.  I then attach the pouch to the paracord. 
To keep it snug, I'll use shock-cord looped through the rivet holes and around the front of the pouch, then through a cord-lock.
I have been using pouches by HSGI and ATS.
Here's a link to a picture over on Bladeforums.
http://www.bladeforums.com/forums/showth...30#post11347930It's a NMSFNO in Mashed Cat Kydex with an HSGI Mini Radio pouch attached to the paracord with Tactical Tailor MALICE clips.  That's two pieces of green shock-cord to hold it down to the sheath really tight (not necessary in this instance) or to hold something else.
The same size pouch can be attached to a smaller knife in the same way or the small knife can be put on the front of the pouch.
After having a Spec-Ops Brand Combat Master sheath, I then decided to use the method described here with the Spec-Ops Brand Modular sheath, which is made for MOLLE pouches.  They should make it in more sizes, i.e. for 10" and 13" knives. <img src="/ubbthreads/images/graemlins/wink.gif" alt="" />
Good thread.  <img src="/ubbthreads/images/graemlins/thumbup.gif" alt="" />