While I'm not completely sure I'm doing gain staging correctly, here is what I've been doing that seems to be working:
1. I use PSP Audioware's VU Meter set so that 0 corresponds to -16 or -12 (lately -16 for more headroom)
2. I place the PSP VU meter as the last plugin in the chain and make sure it's onscreen so I can read it
3. With no plugins enabled other than the meter, I use the track's gain to get the level so that it is hitting 0 on the PSP meter (-16)
4. One by one, I make sure that the output of each plugin shows 0 (-16) on the PSP meter so that each plugin is fed the 0 (-16) level;  If I have 4 plugins on a track I enable the first, make the level 0 (-16), enable the first + second, make the level 0 (-16), enable the first second and third, make the level 0 (-16) and so on.
 
When I do it this way, I have lots of headroom on the Master Mix and can use my mastering plugins without overloading anything.