The best methods for disassembling the housing will vary from model to model. I have a HID ProxKey 1346 Proximity Keyfob. This model has a seam around the middle where the two halves are glued together. So I initially tried to pry it open with a screw driver and then a knife. Unfortunately the seam was too shallow and the plastic was too soft for this to be effective and I only succeeded in scratching up the surface.
The best method that I found for opening this kind of housing is to crack the glue seam by squeezing it. I wrapped the key fob in a napkin to avoid scratching it. Then I squeezed it at one end with a pair of pliers perpendicular to the seam. This caused the two halves to separate slightly at the end. I stuck a screw driver into the opening to hold the two halves apart. Then I repeated this process moving down the side of the key fob until the two halves where mostly separated. At this point I was able to just pull it apart the rest of the way with my fingers.
Alternatively, you can cut the housing open with a knife or rotary tool. If you do this, then you need to be very careful not to cut too deeply or you risk hitting the chip.