Dutta,
Apart from BALHDR and BALDAT - you can also look at the request tables which can be archived. As for the actual process in itself - you would first have to :
1. Identify what BW data is stored in BALDAT and BALHDR - it has information even on IB settings and IB Broadcastings done. Youu can start by looking at each subobject in BALHDR and trying to understand what that means.
2. Given that you have this data - decide what data is actually critical to your needs - in some cases - you might not need Information Broadcasting logs at all
3. Test it out in a staging system to see if you have any adverse system impacts
4. You would need business / user confirmation about the same and get a buy in on time period of retention
5. Enforce the policy agreed upon by way of periodic jobs etc