Whilst trying not to feed the fire here I have to disagree somewhat. Becoming more ticklish is not necessarily a physical change but a neurological one relating to the stimulation of nerves and the reaction. Now that said I very much doubt it's possible to do through a spell, spells rarely give a noticeable instant effect and tend to work more subtly than this would require so my theory is that it'd probably need to be trained in a more meditation type fashion.
Now I actually don't know if this would work or not but it's vaguely based on science rather than magic. If we look at tickling I'd first note that you can't tickle yourself so this is definitely going to require a partner to achieve. Now typically tickling is a friendly gesture and so I imagine it'd help to be comfortable with the person and indeed in yourself and surroundings. Oils can help enhance sensitivity on the skin so I'd probably start out with regular massages, this helps to increase circulation to the skin as well as helping to relax you a little and whilst I'm on the subject of relaxing and becoming more sensory aware let's circle back to the meditation point again.
Then I reckon it's a case of practising, use gentle actions, perhaps tools such as feathers or soft hair brushes. It also helps if you're not expecting to be tickled, typically it's a response to a surprised stimulus so if possible it might help not to fully plan tickling sessions and to try and be taken off guard by it.
Again I can't say any of the above will work and I imagine it would never make you super ticklish but it may help elicit some response.