If you're carrying a backpack... forget anything behind the back... or depending on the pack, you'll probably find anything from the belt that rides high to also be a problem.
I've found that a kydex sheath built to hang from the shoulderstrap of my pack to be ideal - I have a Wegner made sheath, inverted, made to slide onto the packstrap that goes over the shoulderstrap... for over a decade, this has always gone with me and I have never had a problem with retention or deployment.
I also carruy an Eric Berglund puukko in a traditional sheath that dangles from my belt... it extends well down from my pack's waistbelt, so I have no probs with access to it, either...