the_redbeard
Explorer
For some time now, my group has used small plastic chips (like those in a Yahtzee or Axis and Allies game). Green means concentration, white means positive effect, red means negative effect. Place a chip under the mini as necessary. It's not perfect, but it does help a bit, especially to remind a character that already has one concentration spell active to not cast another. Oh how many times I've done that as a GM...
I used chips like that in 4e, but it slowed the game down with so many status effects adding little things to the minis.
I still use chips, but now just put them on their character sheets. I also use them for inspiration.