I can see a few ways that this could work without breaking thermodynamics too badly. But you'll never get "Shields at N percent" with any kind of magnetic or other field that we know of today. This is strictly handwaving.
* The shield acts like a blob of heat-conducting, viscous liquid that slows down bullets and absorbs heat from lasers and other energy weapons. The rate at which absorbed energy can be dissipated depends on the size of the shield. (A "damaged", i.e. hot, shield will glow red-hot.) If it gets too hot, various bad stuff might happen: Generator melts, shield goes critical and dumps all the energy both inwards and outwards, your choice. (This is the approach used by <PERSON> in the Mote in God's Eye).
* The shield consists of an array of targetable force-field projectors -- something like a CIWS with the Half-Life 2 gravity gun or something. If it has to deflect too many threats in too short of a time, they will run out of ammo / stored power / fuel / whatever and the threats will start getting through (gradually at first, hopefully the targeting system will prioritize.) (this is possibly the Mass Effect approach)
* The shield substance behaves in some ways like a normal mechanical solid armor, only it's not made of of ordinary solid matter. 898
It gets damaged much like real armor, dissipating the energy of incoming projectiles (by shattering) or directed-energy weapons (by vaporizing or heat-sinking). If a hole opens in it, you're vulnerable.
* (possibly most realistic one) The shield consists of an actual liquid held in some kind of forcefield (ferrofluid, maybe?) and serves as self-healing / flexible inertia and ablative armor outer-layer. (This will be much heavier and thicker than real armor and probably work best on very large ships in zero-G). Getting hit vaporizes or splashes the liquid, and if too much of it gets used up, there isn't any more.
I think that the idea of shields having a percentage that goes down when they deflect damage is mostly a concession to video and tabletop gameplay where having hitpoints is useful. In movies and the like, they tend to be much more all-or-nothing, but tend to let at least some threats through... which can damage the shield generator... leading to the shield failing. Still not very realistic. 898