I think most people start from the bottom of the legs and work up but there is no reason really for doing this, everyone works out what works better for them in the long run.
If you are ending up sticky then either the wax you are using isn't really that great or, if your clothes are getting sticky, then you are working the wrong way around. When doing the tops of legs start furthest away from you and work towards yourself so that you are not leaning on a bit you have already waxed therefore getting any residue on your clothes.
Personally I start on the backs of legs, start from ankles and work up, then turn over starting from ankles again and work up then over knee with leg bent and I also do the knee end of the thigh with the knee bent for stretch. As long as the top of the thigh is stretched/supported enough, you can get the client to help, you should have no problems.
It may be worth booking a couple of hours with a local trainer to go over full leg with you my darling, it is worth it in the long run and would save you time and save your nerves! :wink2: