Never seemed to be rocket science to me. Mostly, vasodilation is key. This explains:
Why a warm environment or the application of heat to an area will increase sensitivity (blood vessels dilate to release body heat when you're in a warm environment).
Why pot works (THC is a vasodilator, hence the characteristic reddened eyes of pot smokers).
Why feeling stressed can result in a decrease in ticklishness (adrenaline is a vasoconstrictor, and vasodilation occurs in the absence of cortisol, a stress hormone).
Why alcohol (in moderation) can help (alcohol acts as a vasodilator, and its depressant and disinhibition effects can reduce stress responses).
Why ticklishness increases after orgasm (the "flushing" that can occur during orgasm is a visual indicator of vasodilation).
Why poison ivy, mosquito bites, stinging nettles and such are irritating (various allergic and immune responses provoke the production of histamine, a vasodilator which accounts for the inflammation around the affected area).
Why exercise can help (raises temperature which increases likelihood of vasodilation to rid one of excess heat. Also explains why during an active tickle session, a 'lee can feel more sensitive as time wears on).
So, to review: It helps to have a calm, warm environment with someone you trust (decreases stress), and maybe a little something to enhance mood, like pot or a couple of drinks. Beyond that, careful application of heat may be useful -- this is often done with the feet.
This writer does not recommend recreational use of prescription vasodilators, despite some having been used by people recreationally in the past, most popular among which were amyl nitrites or "poppers", said to produce a feeling of euphoria, uncontrollable giggling, and sometimes an increase in sexual desire.
This writer also does not advocate the deliberate application of histamine-inducing materials to create an inflamed, extra-sensitive area.
