I am by no means, a guru, but I do a lot of carved cakes and most of all the tips I pick up from here on CC.
I always carve when the cakes are frozen. If the cake thaws too much while I'm carving, I'll refreeze and wait again, because if the cake starts thawing too much, it can start moving and my knife will have a tendency to push the cake instead of cutting it, which can deform the shape and make it really hard to get the shape you're looking for.
The best way is to work kinda like wood sculptors work--go for the big shapes first, then work on smaller ideas. If you're doing, for example, some sort of animal, rough out the big blobs for the head and body, then work on carving away some of the body to form the limbs and some of the head away to form the right nose, etc.
It's almost always better to take away cake than add it on, so it's a good idea to kinda "whittle" down the cake if you're not sure of where to cut--just start cutting layers away a bit at a time until you can start seeing the shape you're going for. Once you have done a few cakes, you'll get better at visualizing where to take cake away, so you won't have to whittle as much, but there's no harm in going slow if you're not sure. You'll also start to get a better idea of what kind of shape you need to start with--i.e. start stacking squares to get a rough outline of a truck instead of a big rectangle--so that you waste less cake when you carve (although the scraps can always go to make cake balls!)
If you really do have to add on (sometimes, for small protruding objects like ears or the smoke stack on a train, it's far better to carve a small piece of cake to the right shape and then add it on), then use something to glue the pieces together--I usually use buttercream. Push the pieces together really well, then freeze again--the pressure from you pushing on the piece will have thawed the cake, not to mention that freezing will help set the buttercream faster so you can carve on the new piece without knocking it off the bigger cake.
If it's a really complex 3D design (or if you're really scared of not getting it right), it can help to either print out pictures or sketch pictures of what you're carving, especially pictures that are exact front, left or top views (multiple ones are even better, because you can compare the cake to your pictures as you go).
I wrap my cakes right from the pan into saran wrap and freeze. I put a crumb coat on the cake before it thaws, and stick it in the fridge to thaw and "sweat" (which it really doesn't) then I apply the final coat of buttercream. Wrapping my cakes in saran wrap keeps them moist, but frosting them before the cakes thaw is the key to keeping the moistness in.