I find this, too. Many jobs we do require dipping the shoulders - bending down to pick things up or reaching across the table to wipe it or put something down/pick something up.
The straps can be tightened, but if you tighten them too tight, your boobs fall out the bottom!
The silicone non-slip pad sounds a good idea, but doesn't it stick or pinch the skin?
Do you remember those tiny golden safety pins which old ladies used to keep all straps together (in those days, you would have a bra strap, a vest strap and a slip strap all getting tangled underneath your outer clothes!)
Once you have adjusted your bra to the optimum position (I must do this before I get dressed this morning!), I suppose the best way to keep it like that is to (a) never wear it again or (b) if (a) can't be avoided, never wash it again and (c) keep tightening it.
Till your boobs fall out the bottom.