ice 20min on, 2 hours off for days...wear a low profile wrist guard constantly (including sleep) for 2 weeks. get someone to completely strip out the muscles in your hand, wrist, forearm. begin eccentric exercises in all directions (flexion, extension, supination, pronation, ulnar and radial deviation) at low weight so you are sore after 20-30 reps. switch to concentric exercises.
get a hammer with vibration dampening, possibly wear the wrist guard during hammering. continue with stretching and muscle stripping. take anti-inflams for a week after the pain is not acute.
if you experience nerve pain or constant numbness/tingling, or a noticeable reduction in wrist strength you may need less conservative treatment...but start with this.
don't expect recovery anytime soon - if it's really chronic, it could take a year of rehab.
also, check your form while hammering. use larger muscle groups including your core (torso, some shoulder, elbow) and follow through with your wrist.