In what conditions are visible Wingtip vortices created?
You're definitely right about humidity, but from experience I'd say a colder environment works better than a warmer one. Case in point: unstable/rainy autumn weather (as common in Northern Europe) shows (tip) vortices very well, whereas I haven't seen them so much in hotter places (Southern USA, Caribbean). Having ensured visiblity, a larger pressure difference between lower and upper side of the wing (caused by higher speed and higher lift force) increases the tip vortices in size/strength. This explains the fact that (wake vortex) separation minima are largely dependent on aircraft mass (which scales with lift).

Additional (or stronger) vortices can be created artificially using vortex generators. As turbulent flow separates less easily than laminar flow, these are used (on wing leading edges, engine cowlings, tailplanes) to trigger turbulent flow and thereby prevent/postpone separation. This is especially important if there are control surfaces (such as ailerons or a rudder) downstream, which lose their effectiveness in separated flow. In theory, you could use this same principle to increase vortex visibility. 

Whether this also works in theory of course depends on safety standards and regulations concerning modification and the size of the vortex generator required. Examples from the world of sports aerodynamics, where surface roughness or 'transition strips' are integrated in suits worn by speed skaters or cyclists. As you can see on the images in the linked article, these are fairly small.