The Wellhorse method

1. select your intended soon-to-be-glowing material and make your way to the Edit Material box via the Entity Info box. set Opacity somewhere between about 60 - 99%. higher values makes your end product brighter, but of course less transparent. 100% won't work, that just makes it a normal surface again, subject to the darkness we'll let fall in the next step, whereas any below 60% becomes increasingly too transparent and also looks dark.

also, use colors that are already bright, like yellows or whites. dark blue just can't be made to glow like bright yellow can!

2. make sure Shadow Settings are enabled, like Wellhorse mentioned. change your time and month to something as dark as you can get it, but stop just short of the end of the scale.

3. last but certainly the most important, change the Darkness color to 100 percent white. this seems to be the key in getting your whites whiter! i don't know why transparents still react to this even after the sun has set, but react they do!