Clap your hands twice as though the lights work on a clapper. If the lights don’t come on straight away the switch should work now.
I have done this successfully on 3 separate occasions, please try and let me know your results!
WARNING: This has a high chance of changing your dream scenery.
What about setting everything on fire? 
I have done this successfully on 3 separate occasions, please try and let me know your results!
Wouldn’t commanding the light to switch itself on be a good idea?
Though it might just decline and talk back to you.
Wow, I haven’t been here in ages. What I’ve found is that the only reason something in a dream doesn’t work (light switches, reading text, ect.) is because people read that it doesn’t work, so they expect it not to. For example, the warning that there is a high chance of the scenery changing if you clap your hands. The only reason that any dream’s scenery will change by doing this is because people read the warning, and expect it to change. So therefore, just using the switch should work the same as it does in real life, as long as you expect it to.
This is only what I’ve found from my own experience, and might not apply to everyone though.