I imagine that they are made invisible by producing a field around the body that bends the light around them so that they are invisible(in case anyone didn't know a scientist made a "invisible" cloak...so a lot of this stuff isn't really far fetched...other than the fact that they are superpowers are latent human abilities) :P as for DL phasing through materials but still having clothes on, I think it's sort of the same principle but with phasing/molecular dispersion... he produces a field close to his body but able to envelope his clothes so that he phases through items while clothed... his abilities are just like Shadowcat (Kitty) from X-men... she's not naked when she phases..though I don't think anyone would complain if she was... but we got to keep things family friendly.
From wikipedia about kitty's powers(she's way cooler and cuter than DL, but he's more realistic). :P
Kitty has the ability to phase, or flow, through solid matter by passing her atoms between the spaces of other particles through which she is moving. Using her power was optional, but for a period (over ten years of published comics, approximately two years in-continuity) Kitty existed in a naturally "phased" state, and had to consciously choose not to do so. (However, she returned to her original form and is now naturally solid and must choose when to use her power.) While phasing, she does not physically walk on surfaces in this form, but rather interacts with the molecules of air above them, allowing her to ascend and descend. While phased, she is immune to physical attacks and most energy attacks, and has a high resistance to telepathy. The density of some materials (such as adamantium) can prove deleterious to her phasing, causing her to be severely disoriented if she tries to pass through them. Magic and magical beings also prove to be an enigma in regards to Kitty's phasing, as proven in a battle with a N'Garai demon that showed it could cause physical harm to Kitty even while in her phased state, as well as the Soulsword. The use of her abilities also interferes with any electrical systems (including, in some stories, those within the human body) she passes through by disrupting the flow of electrons from atom to atom.
Kitty can also extend her powers to phase other people and objects. Currently, she is able to phase at least a half a dozen other people (or objects of similar mass) if they are in contact with her prior to her phasing. Her defenses and form of movement extend to those others she affects.
Besides her mutant powers, Kitty is a genius in the field of applied technology and computer science. Since her possession by Ogun, she has been consistently shown to be an excellent hand-to-hand combatant, having since been trained in the arts of aikido, karate, ninjutsu, and Krav Maga. She also has training in gymnastics and is a professional level dancer.