I suspect that the most common inhibitor of poor tone among beginning to moderatly advanced students (and likely beyond that) is not playing with an "oral cavity" (as Dave mentions) that is open enough. It's almost always the cause in my case when I notice poor tone starting to creep in. If you don't play with an open enough oral cavity and palate/throat region, your tone will lose resonance, get thin or stuffy, not have a clear "center", and will sound kind of "pinched" and sort of wander around in pitch.
This may sound like an "obvious tip", but it's so prevalent that it's always worth considering. It's often accompanied by a pinched or otherwise malformed embouchure, and often caused by trying to "force" a particular pitch or dynamic.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)