Pretty much what Dave and Rick have said ...
I do NOT like wrestling with any instrument, so I strive to hold the instrument with as little muscular effort and tension as possible. That solves a number of problems. For my large EEb tuba, I built my own sit-on stand that completely supports the instrument. For my euphonium, I made a "lap board" (padded with the waffle-pattern rubber shelf liner). There is no sense in holding a horn up with muscle power if you don't have to -- especially if you're older and maybe have arthritis. Just yesterday I soldered a finger hook onto my bass trombone to relieve what was otherwise just too much gripping force required (a ubiquitous problem with bass trombones). Effort and focus should go into playing, not fighting, the instrument.
And there can be effects of fighting the instrument that you don't don't want. For example, one thing that may be causing you to have problems with friction/wear on valve stems is that you're having to hold the horn in such a way that instead of your fingers just sitting on top of the valve buttons and pushing/releasing straight up and down, you are (perhaps ever so slightly) gripping the valve button with your finger over an edge (or pulling the valve button as you push it down) -- and this because it's part of how you're struggling with the horn. Keep an eye on things like that because it can really screw up your pistons with uneven wear fairly quickly.
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)