It is not easy to get over here.
From Cairo there are daily buses to Marsa Matruh or the Libyan Border stoping at Matruh and from there you have to wait for a bus or charter with some other travellers and locals a minibus to get down to Siwa.
Donkey is the only mode of treansport aroun the town. You are better off with a bike. But when you are in Siwa, do the Siwans do! I just had a short ride with it.
Change bus at Mersa Matrou. It takes you almost 5 hours from Cairo to here and then another 5-6 hours to Siwa. Well...it worths!
rent a bike and enjoy of a pleasent ride trough the palms and olive trees, stop to breath at the top of the Oracle or the Gebels and swimming at the pools.