D&D 4E gamers are always trying to figure how best to capture the many conditions and marks that the game imposes. Marked, Divine Challenged (itself a mark), Cursed, Oath, Quarry, Ongoing, Dazed, Restrained, Grabbed, Slowed, the list goes on and on!
We usually start with just paper notes or our memories and find that dissatisfying. We find we forget what is affecting our critter (our PC or, if we are the DM, our monsters). And, it is really easy to forget what we did to others.
We then usually come up with some marking system, such as miniature clothespins, soda screw-top rings, wooden tokens, and so on. But, we then often find that too much marking becomes its own level of clutter. With so many marks, we can lose track of everything. A creature can be dazed, bloodied, slowed, have a -2 to attacks, have a power bonus to damage, and be suffering from two types of ongoing damage.
Of late, I see two sound techniques.
Whiteboard and Reference
The first is to have a reference system linking back to something that holds all the info. For example, use something to mark figures with a color. Then, on a separate place, such as a whiteboard, track all the details for all the creatures, separated by the colors. So, you have a mini and it is tagged blue. On the whiteboard, it reads "Marked by X, ongoing 5 (S.E.), slowed (SE), -2 atk" or something similar. This can work. The board needs to be near the DM and close to everyone. Ideally, you develop a system to minimize writing, such as using a magnetic board with tokens for common conditions or having some shorthand everyone learns.
Marking Bands for the big effects
This is my current approach. Here, each PC (and foe) that can mark something in some way as part of their class gets their own color. The paladin might use white, the warlock grey for curses, the warden brown. The DM has a few colors for common conditions important to the encounter. For example, a fight with a green dragon might necessitate stunned, poison 5 and slowed, slowed, poison 10, -2 atks. DM simply explains the key as they deploy the bands. "Ok, guys, the dragon has breathed. Everyone with dark green is slowed and has ongoing 5 poison. When you save, I'll give you the light green, showing you are slowed."
You generally don't mark minor things, such as conditions from a power that will not be used over and over again and which will only last until end of the next turn, since those can be remembered well enough.
For marking, I started being cheap and using the rings that are left around the neck when you unscrew bottles of soda, but some players felt they obscured the minis too much. I've switched to their method of using Goody's elastic hair bands. They come in various sizes. The medium is about the radius of an index finger and work great for medium minis. You can use the small ones as well for many minis. The large ones work fine for larger minis. They have multi-colored packs of 160, which are more than medium-sized rings for all your needs. I bought two packs, one of each color combination. The colors are a bit pastel, but this actually works well since the bright color contrasts with the paint of most minis.
A word on responsibilities
Especially with the above system, it is important to place responsibilities appropriately. Everyone should take responsibility for remembering what they have done to others. That is generally the hardest part to remember, so it makes sense to place the responsibility on the inflicter. If you placed a Rattling effect on two foes, you should remember that! Some effects can be pretty specific/conditional, so it makes sense for those knowing the rules to be in charge. Secondly, you try to remember what was done to you, but this is a safety valve and not your main responsibility.
For example, we might mark that the dragon is dazed with a marker, but not that it has a -5 to attack the invisible rogue. The rogue's player is the one that tracks that, since it is an infrequent power and the rogue can best track it.
By using a few rubber bands for the big things and having the responsible parties track the other conditions/effects they grant/inflict, the game becomes much easier to monitor and time can be spent on better things (such as describing IC what you are doing to that poor solo monster).
What do You Use?
Edit: Sly Flourish has a video blog on options here.