It sounds like you want the rules for Warding and "land mines", except that instead of trying to keep something magical out of your home, you're trying to keep it out of, well, yourself.  Maybe you could adapt the Ward guidlines to an enchanted item?  Instead of using the Ward as the baseline complexity, you could use a Reflexive Block, and then add land mines on top of that.

Consider that as far as DFRPG is concerned, what you're really doing is using magic as a substitute for an Investigation roll.  Being magic, it makes impossible things possible, etc.  But even given all that, and assuming it was possible to locate a person or piece of gum in a city ... which would be easier?  I'd tend to say that even if it was possible to locate gum somewhere in the city, it would be in the legendary range. 

Well, my intent in this post is to find a way to magically "bug" someone without them knowing.  So I was thinking of the gum (or similar objects) as the magical version of a radio transmitter, which would be relatively easy to locate in a city.

Keep in mind, by the way, that all of these systems are extremely subjective, and these are my opinions.  Your mileage may vary.  The most important things to consider are (a) fun, and (b) game balance -- and remember that anything you convince your GM to let you do is more than likely to be done back to you at an inconvenient time.  So it's possible that undetectable symbolic-link-aspect evocations capable of guiding TMDs (Thaumaturgies of Mass Destruction) are not a good idea. 

Agreed.  I think it might be fun to add some espionage to the Dresden Files (tracking spells, reverse-tracking spells, discovering that your enemy found the "tracker" and set up an ambush, etc.).  But I wouldn't want to start using it to fire magical nukes at people from a distance - that seems so undignified!

Here's my idea for marking someone on a budget.  Note that it is not spectacularly sneaky, but could work against a subject that was not actively looking for threats in a sufficently public place.  A party of some sort would be ideal.

Get yourself some chewing gum.  Chew thoroughly.  Bite off a small piece and put it in your hand.
Go say hello to the subject.  Give him a hearty handshake, quickly pressing the gum onto his arm or shoulder with your other hand.
Take the remaining gum to your arcane sanctum and attach it to a picture of the target.  You now have a symbolic link that satisfies the magical laws of contagion (the link between the two pieces of gum) and similarity (the picture as representative of the subject).  That's +4 toward the complexity right there...

If you were just trying to track or scry on the gum (because you knew it would lead to the target as well), would you need a picture of the target?  Would that lower the complexity of the tracking spell, since you wouldn't have to overcome the gum's Conviction?  (This assumes that you didn't use a truly righteous gum, of course...)

What are some different ways to "mark" someone so that you could track them later via Divination?  Basically, I'm interested in finding a way to sneak a symbolic link onto someone.  (The most obvious way is to sneak a physical item into their pocket or something, but I'm looking for something that doesn't require Deceit or Burglary.)

Could you mark someone with Thaumaturgy?  In other words, could you cast one spell to create a symbolic link, and then use that link later for other thaumaturgical spells (attack, scrying, tracking, etc)?  This would sort of be like hexing someone, except instead of giving them an Aspect like "Bad Luck", you would give them an Aspect like "Magically Marked".

The Intent here is to mark someone while they are physically present (ideally without them knowing about it) so that you can use Thaumatrugy spells (like Tracking) on them later when they think they are safe.

Some possible ways to mark someone:
- Get them to drink a magically "spiked" drink.
- Create a "land mine" that marks them if they break one of your Wards
- Create an enchanted item that supplements one of your attacks by marking the target

Would any of these be possible, or is it outside the mechanics of Thaumatrugy?

If it *is* possible, what are some other good ways to mark someone?

I started my campaign and I have one Wizard in the party. So far he throws around fire smartly enough but the thought occurred to me. What other element could he want with an attack when fire covers 9/10 harmful substances to throw at monsters?

Assuming he chooses refinement when he gets a chance. Why would he want to choose another element?

I suppose it depends on the kind of consequences and aspects you might want to inflict on people.  It would be difficult to justify a Grapple maneuver with Fire evocation, for example.  And most consequences are going to be harder to hide or explain.  For example, "Nasty Burns" might raise more questions at a hospital than "Broken Ribs".  And if you're just trying to stop some runaway kids who stole your Blasting Rod, do you want to throw Fire at them, or is it better to take them out with something more subtle (like tripping them with an Earth Block)?

Fallout from a failed Evocation will be very different as well, depending on the element you use.  Fire evocation fallout will tend to set things "On Fire".  Air evocation fallout might knock "Everyone Prone" or kick up "Lots Of Dust".  Spirit evocation fallout could give everyone "Splitting Headaches".

I'd be tempted to run it straight up as a Veil, with limited control by the caster.  That would make it an Evocation block versus their Perception and should draw off attacks.

That's an interesting idea.  How would you actually run that?  A block limits someone else's actions, so I'm not sure how you can link it to Perception.

Round 1: Wizard casts "Duplicates", with a Block strength of Superb
Round 2: Ogre uses Fists to attack

In theory, the Block would be against Fists, not Perception, right?  I guess I'm not clear on how veils (and Stealth in general) would work in Combat.

Stunts and Powers:
+2 Green Lantern Ring (Obvious during use, glows green, can't hide the light)
* +1 Feeding Dependency (The ring is powered by life force/magic. It can feed off the user with mental and physcal consequences, or it can be used against other living or magical creatures to feed off of them)
* -2 Channelling (Green Force Constructs, +1 Power and Control)

Does the +1 Power and Control come from the Focus Items aspect of the "Channeling" stunt?  (I think so, but at first I wasn't sure where the bonus was coming from.)

I'm mildly concerned about the size of the Item of Power vs. its abilities.  From page 281, "Hidden Power", a ring could only hold 1-2 Focus Item slots or 3-4 Enchanted Item slots.  The "Channeling" stunt would fill up the two Focus slots by itself, even before the other abilities were added on.  From a "Dresden Files" standpoint, you might be better off emulating something like Starman's "Cosmic Staff" http://en.wikipedia.org/wiki/Starman_(Jack_Knight).  Something that powerful should be (relatively) easy to disarm or pin down, for added drama (and Fate points).

I really like the idea of a Feeding Dependency, but I'm not clear on how you've designed it.  It doesn't look like it follows the same rules as the Vampire Feeding Dependency.  Is it enough of a disadvantage to warrant a +1?  How is feeding off the things that you are already attacking a disadvantage?  Does channeling require two mental stresses (one two use the ring, and another to channel something)?

Overall, cool idea!