There's normally 3 buses an hour from outside the HGI to Luton station, 2 quick ones, 1 slower one that goes via the Airport and past Luton Airport Parkway first. Add "PlusBus" onto your train ticket for about 2 quid and the bus ride's free.

Otherwise, surprisingly (and you may need to print out the map to prove it), the Hitchin PlusBus zone reaches out to the HGI Luton as well. 2 fast buses and 1 slow bus an hour to Hitchin town centre, 10-ish minutes on the quick ones, then a 10 minute walk or change buses to get to Hitchin station. Can be handy too, depending on which way you're heading by train.