Usually via the Mithras House Park & Ride - handy because it's possible to get back there by regular bus (5/5A and change on to the 49 at St Peter's) if a post-match drink in the Sportsman is needed.
Occasionally train from Glynde (free parking) or Lewes, then walk to the Lord Nelson and 5/5A bus from St Peter's.
Have even been known to drive to Withdean mid-morning, catch the shoppers' park and ride into Brighton, do some shopping, visit Nelson and then travel back to Withdean on the 5/5A.
Once drove a bus, parked it at the Brighton & Hove Bus Company depot at Lewes Road, and walked back to Mithras House park & ride. But that was a one-off.