Bullet sprite can be as large as is reasonable (maybe larger)
Can be mostly white or some other striking colour, fading to another colour near the edge. SnoutUp posted some wonderful examples of this.
Bullets can move ridiculously fast to start, and lose speed over time, expiring when they stop or just before (only if they're the player's bullets. Enemy bullets should be slow or there should be adequate warning before they're shot)
Increase the number of bullets. Instead of one bullet, shoot five at a time.
Increase the rate of fire. Instead of five shots per second, why not twenty?
For a frame or so when the bullet first appears, the entire screen can be slightly illuminated. Make colours lighter for the briefest fraction of a second.
Shake the damn screen as much as you can get away with (then shake it more)
If you have the skill, do some chromatic aberration.
Freeze the game for 0.01 seconds or so every time they shoot.
Zoom out the camera while the player is shooting.
If you can do a lighting engine (or buy one or whatever), you should definitely have a vicious flash of light with every gunshot.
etc.
Here's some more talks on the matter: