Here's some things I do to make my lucid dreams brighter.

If it's dark inside:
- Expect the lights to turn on by themselves.
- Command the dream to turn the lights on.
- Find a light switch and "know" the lights will turn on.
- Open a door, fully expecting it to be light outside. Visualize as you do this.
- Commandingly shout something like "brightness now!".
- Sometimes light switches do work. Again, it's just a matter of expecting them to work.

If it's dark outside:
- Find the moon and drag it down; dream may automatically make the sun come up to replace it.
- Commandingly shout something like "brightness now!".
- Expect the street lights to turn on.
- "Realize" that it's dawn and the sun is about to come up. In the dream, believe it'll come up very fast and won't take forever.

I've done the above quite a few times but sometimes, even though the lights turn on and the sun comes up, the actual lighting in the area doesn't change. This is why it's important to visualize how shadows will appear and how the objects around you will actually get lighter.