Photographing fluorescent (day-glo) colours is difficult because they are much brighter than they should be. They are actually light sources in the visible spectrum (they absorb invisible ultraviolet light, then re-emit the energy as visible light). It's not that they aren't bright enough, but that they're too bright, and if everything else in the image is exposed properly, they will be overexposed. Even if you dial back the exposure a bit, there is a good chance that at least one colour channel will be blown out, since the fluorescent emissions are at very specific wavelengths. (This is quite different from ordinary dyes, which re-emit over a rather broad spectrum and essentially turn all of the light that isn't the right colour into waste heat.)

You can try a few things, most of which won't be entirely satisfactory. The first is to underexpose as much as you dare to bring the fluorescent colours back into a usable range, then try to raise the rest of the colours to match. That will probably result in an unacceptably noisy image. Conversely, you can try to selectively bring the fluorescent colour down in post, but given that one or two channels will likely be blown (fully saturated), you will probably wind up with significantly less detail than you were hoping to find -- but with just the right colour and careful post-processing, you may be able to grab enough luminance information from an un-blown channel to arrive at a realistic rendering.

Both of those assume "action" shots -- real-life people who are wearing the clothing. In studio, as product shots, there's another option. That's to take two photographs. One should use a light source that has little to no UV (or near-UV) content -- filtered flash or tungsten will do the trick. (Fluorescent lights usually have too much UV content for this exposure.) That will give you the shape, drape and texture you need. The second exposure will use a high-UV source (unfiltered flash, fluorescent or "black" light), exposed to get the tone at or just above a middle value. That exposure will give you the garish colour, and not much else. You can then blend the two exposures using any layer-capable image editor. (You can also try colourizing a single non-UV exposure, but it's hard to get made-up colours right.)