Sometimes a matter of inches can make all the difference. In winter steelheading, sinking a fly another foot can mean hooking a fish when the guys in front of you got blanked.
To sink our flies, winter steelhead fly fishermen use sink-tips that range from 2 to 15 feet. When the water calls for it, we add a weighted pattern. And when the run is especially fast or deep, we can slip on a cone weight to the leader. But sometimes even these tackle adjustments leave us short, or shallow as the cause may be.
When I want to sink a weighted fly as deep as I can, I change the angle of my cast; instead of casting at a target on the horizon, I aim at a target about 30 degrees above the horizon. This angle change causes the fly to hinge at the end of the cast, then drop fifteen or so feet straight down into the water--where it plummets thanks to the acceleration of the drop AND the lack of tension that exists in a typical straight line cast.
To get even deeper, try making your mends in the first seconds after the cast lands; these mends will be most effective if they don't add any tension to the line. Try retaining a loop in your line during the cast (as the cast tightens and drops, you still have a big loop in your fingers); then release the loop and throw the mend.