I agree with Satya's suggestions, especially playing real time sports with lots of people to keep track of. That will force your Se to take over and push your other functions aside.
Here is a fairly good source of information on
Extraverted Sensation.
Se focuses on the here and now. It focuses on the immediate senses - sight, sound, smell, taste, and especially touch. The best way to develop Se is to focus on your senses, as they are in the present without considering the implications or connections to the future, the past, or associations to anything else. Se is the ability to be fully in the moment.
Drawing, painting, dancing, music, and a whole variety of other artistic endeavors that involve non-verbal channels are great ways to help develop Se. Sensual massage, and martial arts sparring are also great for developing Se. Archery may be the single best Se builder available, with target shooting running a close second. The most important part of this approach is to make sure you are doing something that overwhelms you and requires you to 'not think' to succeed, forcing you to stay in the moment and focus on what is before you and around you. It has to be something you 'just do' so that your Se takes the dominant role in the activity. If you can do the activity with your Ni, Fe, or Ti, you'll likely use them first and develop your Se much less than you would otherwise.
This might seem like a monumental task, but developing Se is the best way to help gain control over your Ni, which is a constant source of distraction and temptation to get lost in one's imagination. This is a highly recommended ability for all INFJs.
Tactics and strategy are the realms of
Introverted Thinking, which is the partner of Se for INFJs. Playing strategy games will also help develop Se, but only by proxy. You'll be developing your Ti much more. Other ways to develop Ti include tinkering with puzzles and games that involve solving like sudoku. Developing Ti is also a very good thing for INFJs to do as it helps us gain more conscious control over our Fe and be more rational when things are not in alignment with our ideals, and even help us think our way out of situations that overwhelm us. Craftsmanship of any sort can help with the Ti/Se pair a great deal, especially once you gain enough proficiency with a craft that you don't have to 'think' about it, but can simply get lost in the act of creating.
In both of these cases, Se and Ti are right brained functions. You'll know you're using them more dominantly when you lose all track of time. The right side of the brain has no clock. If you're still aware of the time, you're still using your Ni and/or Fe more dominantly than not.