To deliver the maximum glow possible with glow-in-the-dark inks, consider the following tips:
Don't add colorants. Live with that dull greenish-yellowish-blah color, as adding any pigment will reduce the glow.
Lay down as thick a coat as possible without making the print look bad. An under-base white or undercoat of clear ink may help with laying more ink on the surface of the shirt. Do this even on white shirts.
If possible, use thicker lines in the artwork. Thin lines and type don't have enough mass to glow, so the effect won’t be noticeable.
Do not over-cure the ink. Very high temperatures can damage the phosphorescent properties of the ink.
Leave the ink in direct sun or bright light for as long as possible. A few minutes in low light won't achieve the maximum effect. It depends on the brightness of the light, the thickness of the ink and other factors, but a rule of thumb is to give it 20 to 30 minutes in a bright light to have the brightest, longest-glowing effects.
—Rick Roth, Mirror Image Inc., Inkkitchen.com