If you do not have a table available, here are some ideas on how you can still give your partner a great massage and not get yourself out of whack. Also, make sure and look at the Thai Massage and Stretching videos page for more ideas. I am also going to be shooting some floor massage videos for the site very soon.
The video below gives you some ideas on how you can make your partner comfortable if you do not have a massage table available. Be creative and check with your partner often to make sure they are comfortable.
I think the next video has some good ideas for floor work, although most of it is pretty light. She uses a nice firm pad on the floor, and seems to have a nice space set up. Some of my thoughts watching this video were:
Try to use as wide of legs as possible to keep the work out of your lower back. To me it seems like she might have her tailbone to high around 2:10. She also uses a nice staggered kneeling position around 9:50 which looks comfortable. Watch your posture, keep your center of gravity (weight) over your hands and sink in to your partner.
Around 2:40 she talks about the neck being turned and how you can't work the kneck on the floor. Around 3:20 she talks about feeling strain in her body. If you feel strain, change positions. Try leaning on your side or straddling your partner like in the Thai massage we did on the table in the workshop.
I think this video gets good at about 7:10 when Jen starts working with her partner on her back. She does some nice hip and low back work with some trunk rotations.