How can I prolong the life of my bottom bracket bearings?
Actually, IMO Hicks is right in the comments under the question. Now, had the problem been really that the BB is wearing out:



After you have been out in the wet, on returning to base (home), remove the bicycle's seatpost and turn it upside town. You will be surprised how much water runs out!

You might also want to try sealed cartage BB or external bearing BB. By the way, ordinary (not cartage) bearing are extremely inexpensive: 0.5 euro at my area.