Holy god damn windows 98.
Save often. The planes of Hammer are a dangerous place.
When I use a program like hammer (currently working in unity3d) and it crashes on me, I do freak out to a point where I tell myself, I know what I have just made and it wouldn't take me long to re-make it. Unless of course its a 5000 poly complex organic model in 3ds max then I would cry.
Well, Hammer has an autosave function which you can custom set to X amount of minutes or X amount of actions (changing entity parameters doesn't count). I think mine is set to 15 actions/5 minutes, i never seem to really lose any stuff when Hammer crashes unless i just changed a bunch of shadows/collisions on prop_statics; that can take ages and doesn't count as an action to the auto-save feature (it needs to be brush creation/manipulation or converting/adding entities).