Check it really is sciatica - and have they had a med diagnosis and /or scans, just in case the nerve is trapped by the spine and not just by tight muscles. If it's spinal massage probably isn't going to help, but that is quite rare. In my experience it usually turns out to be tight buttock muscles, and is more prevalent in desk-bound or static people.
I've had success with releasing the piriformis muscle underneath the glutes, after working on lower erector spinae, quads and hams to release whole pelvic area, but this is pretty painful for the client so you need to know the move, and be able to reassure and guage how much pressure to use. Once you've got the technique its actually pretty easy.
There are also ways of releasing the iliacus at the front of the pelvis, which also helps with mobility in this area - but again, get some advanced training (Jane Johnson - London Massage centre and School of Natural therapies in Thames Ditton is really good) as it's not something to learn from books!
Amanda is correct about treating other synergistic muscle group in order to relese the nerve trap.
If you look at the dermatome of sciatic nerve involvement and trace the muscles group concerned, and massage them until they are soft,more flexible and lenghten them by MET, you will get good relief for your client.
If you want to carry out all this without straining your hands and fingers contact me on 01524 843435 or email me siew@jessonclinic and I will share with you my secrets FREE.
I have been doing remedial massage for the past 15 years and it works everytime to understand dermatomes of each nerve pathways. It is like learning about the motorways systems and driving on them becomes second nature. I hope you agree.