Firstly get an early flight to malaga airport, then get a bus from outside outside arrivals, just walk straight out and you will see the bus stop. Ask for a ticket to Autobus Estacion, This ticket costs 2 euros.
The first bus that leaves malaga bus station to Olvera is at 12.30, that is with the bus companyy Amarillos, (you buy the ticket from the Amarillos office in the bus depot) and there is second bus at 1pm, with the Casado bus company.
The bus takes a little more than 2 hours, (with a toilet break in the middle!) giving you plenty of time to go for a coffee and some tapas until the bus from Olvera to Pruna leaves at 4.15pm
The ticket to Olvera costs 11.40 euros (may 2012) and the ticket from Olvera to pruna costs 1.2 euro
The actual timetable is:
Malaga to Olvera 12.30, 13.00, 16.15, 19.00 (all these are for weekdays, Saturdays and sundays have a much more limited service)
There is a bus from Pruna to Olvera at 12.30, then you can get the bus to malaga at 16.00, this should get you to malaga, and then a bus to the airport for ariving at approx 19.30, so with a late flight out there will be no problems (there is another bus at 18.30 from Olvera so allowing a paranoid 4hours to get to the airport that would permit a flight after 23.30)
But there are locals who will happily drive you to Malaga airport for a mere 80 euros or so, so for return flights, cut th ehassle, and get a lift.