Most players consider Support Lux as a troll pick especially at lower ranks but I think she's pretty much viable as a support, she has everything that a Support needs to help their ADC and team. Shields(big amounts of 'em), stun and a slow. You can play her as a Pure Support with little to no damage but tons of shields and cc to cover for your whole team or you can build her damage specially when your team is lacking.

Poke enemies with E but be mindful of your mana, once you get enough mana regen you can spam the hell out of everyone.
Always save your Q for ganks and/or opportune moments to get a kill.
Practice aligning your W even when your ADC or teammate(s) are moving.
Also practice Q'ing through minions for a surprise root and maybe get a kill.