play in a well-lit area. Natural sunlight near a window or outdoors works great, too, as does playing near a lamp, etc. A light source that is above or behind the player will maximize the brightness of the game. Different viewing angles also gives the best results.
The dark display on the Nintendo Gameboy Advance is is a function of the display technology, so nothing can be done about it directly. The easiest solution is to change your lighting environment. There are also be third-party lights that attach to the screen. The most extreme solution is installing lighting under the screen yourself, but this is difficult.
Gameboy Advances have a dark display due to the fact that there is no backlight. To alleviate that, you could buy one of those light accessories that hook up to the GBA.