I got a decent deal on autoglym's leather cleaner and balm from halfords, admittedly it's not the best, but ones the job.
Hoover the seats, getting into all the seams to remove all the dust etc.
Depending on how dirty the leather is, I'd get a bowl of warm, soapy water, and wipe the leather down first, get into all the books and crannies. Use a clean damp microfibre cloth (not wringing wet)
Allow it to dry and then apply your choice of leather cleaner, wipe off with a microfibre cloth and clean warm water.
Then apply your balm or protector, this should tighten and firm the leather up, it might e a bit slippery once dried, or the autoglym stuff is anyway.
If you have shiny bits, I found a good wipe over with a mix of 1:4 isopropyl alcohol and warm water will remove the worst of it.
Clean and re-apply balm 2-3 times a year