Epoxy is the answer
First dry your conker. Drill a hole through for the string, and once it's nice and dry inside, you should manage to rummage round inside it with a small screwdriver such that the innards can be removed.

Now mix up some two-part epoxy, like Araldite. Use the slow-setting stuff in the blue and yellow tubes (not Araldite rapid) as it sets harder. Inject it into the conker using a syringe. Place somewhere warm for 24 hours - an airing cupboard is ideal - until the epoxy cures fully.

You will need to re-drill the hole for the string, as the inside will eb rock hard.
( K2k6 has a proper job these days, Fri 19 Sep 2008, 12:14, Reply)