In general they get there by having a seed dispersal strategy that gets a proportion of the seeds into the right location where they will grow. Orchids, for example, often have extremely numerous fine dustlike seeds, and release them into the wind so that some proportion of them are likely to blow into suitable locations. This is the same technique many fungus and ferns use with their very fine dust-like spores. There are other strategies that ephiphytes might have. For example the common mistletoe found in Europe is one of those plants that relies upon having the fruit eaten by a bird (or other animal) for dispersal. The animal then excretes the seeds out the other end, a proportion arriving in suitable locations. In the case of the epiphytic mistletoe. the bird (often a mistlethrush) excretes while sitting on a tree perch, and some will fall onto a tree branch rather than onto the ground, and even after passing through the gut of a bird the mistletoe seed retains a rather sticky covering so it can stay there. I think strangler figs use a similar technique, though in their case they rely upon the seed falling into a place on a tree that has collected some organic matter it can grow in, eg, at the upward facing fork of a branch - figs have very numerous seeds so only a tiny proportion need to arrive in good locations.