When flying under visual flight rules (VFR), pilots have to be able to see far enough away to be able to avoid running into other aircraft or the ground. Therefore, VFR flight requires good visibility with a high (or no) cloud ceiling; this is known as visual meteorological conditions (VMC). It also requires that pilots avoid clouds like the plague, since clouds (on Earth, at least) tend to be on the opaque side, and a cloud that you can't see through could potentially be hiding something that it would be very bad to run into (such as someone else's aircraft).
Avoiding clouds is easy enough during the daytime, if the weather is good (the big white things tend to be fairly visible as long as you aren't actually inside one), but, in many countries (including such biggies as the US, France, and Australia), VFR flight is also permitted at night. As clouds are not usually polite enough to illuminate themselves for the benefit of nearby pilots, they tend to blend in rather well with the dark sky (except sometimes when bright moonlight is present), the only hint of their presence being that they occlude background stars (which provides absolutely no information about whether those stars are being hidden by a small cloud close by, or by a big cloud further away, and depends on the stars being easily visible to begin with [which is not always the case, even at night]).
So how do pilots avoid clouds when flying VFR at night?