We use the little plastic rings at the top of plastic soda bottles. Red for bloodied, White for Cloud of Daggers, Yellow for Marked, etc.
First off, they are cheap.
Secondly, they can be draped over the mini where they are VERY visible. The mini does not need to be moved with these like it does in order to put a marker or poker chip under it.
Sometimes they fall to the base of the mini, but that does not often happen. But, it doesn't really matter.
The leftover different colored rings we use for conditions. We tend to re-use them for different conditions because a) there are so many conditions, it's problematic to remember a different color for a different condition, and b) conditions happen infrequently (at least at low level), so we can remember the one or two conditions as long as any plastic ring is there to remind us.
If we run out of colored plastic rings, we use colored stones next to the mini for Conditions. But conditions do not happen as much as bloodied, marked, hunter's quarry, cursed, and cloud of daggers in our game, hence, we have hardly ever pulled out the stones.
The main advantages of using these plastic rings are a) it's easy, b) nobody has to write anything down, and c) these elements of the game can be discerned by anyone at the table with a single glance. Since they typically drape over the minis, one does not need to move mini #1 in order to see what color poker chips are under mini #2.
And, if one wanted to make special easily identifiable rings, one could take the colored ones and put black marks on them with a permanent magic marker.