It's something of a shame that somehow typical "spiritual" enlightenment has become intertwined with "solemnity" and the feeling of humour or fun has been replaced with grey seriousness. There is nothing more spiritually (or otherwise) uplifting as a beneficently glowing, heartwarming smile!
So how can you "lighten" (or "en-lighten") your heart? Have you ever seen a smiling Buddha image? Have you ever forced yourself to smile even though you feel angry about something or at someone? This is just my humble opinion, but I think this is the mark of a truly enlightened being: One that can always smile in the face of any adversity. If one is deeply "enlightened", their light shines so strongly that their heart overflows with warmth and mirth even when our "dark" and "heavy" hearts struggle in cold, lonely and fearful places and situations.
It's relatively easy to do in daily practice, so long as the object of your annoyance is cute, like my little "evil" dog, for instance! ;) She is part demon, and as a result, destroys things around the house and has unfortunate "protein stain" accidents. I practice "enlightening" my heart when the anger rises, and yes, admittedly I fail sometimes. Most notably, when my bodily and mental conditions are also poor (egs: I'm hungry, tired, cold, in a hurry, preoccupied, anxious about something, etc.) then I'm predisposed to crankiness and have more difficulty "un-Grinching" my heart. (... remember the Grinch's heart AFTER his epiphany?) Her cuteness protects her most of the time, but when I can feel "light-hearted" about things, her naughty antics don't bother me as much.
It seems like a luxury to take the time/energy to put yourself in a position where you can literally smile and/or laugh in the face of adversity, but I would submit that it's actually much more critical than you may realize. Dealing with a naughty dog is one thing, but dealing with your loved ones, who "ought to know better"!?! Grrr... Why do we say/do that? How does it happen? Do we use up our patience in our busy lives, with our clients/colleagues during the day, then have no time/energy/patience left for the ones that REALLY matter in our lives?
As a teacher, I pride myself on my patience, perhaps a bit ridiculously so. But I realized at one point that the more patient I was in class or at work, the grumpier I was when I got home to my family. While it wasn't a one-to-one inverse relationship, I started to consciously decided to "save up" some patience and take at least some "leftovers" home with me to the people that I really cared about and who truly deserved it. Again, not a perfect solution... Some days, there were no leftovers and I couldn't muster enough "second wind" patience. But I'm getting better at it, particularly when I dissociate (though I know it seems like a potentially dangerously mentally ill word) and simply remember that my family members don't deserve to suffer for something that I collected from someone else earlier in the day.
At any rate, the bottom line is: Do more smiling, joking, laughing... I know it seems ridiculous, but they work... particularly when inappropriate! ;) We've all heard the phrase, "Laughter is the best medicine". But what is this medicine acting on? The simple answer is that it's actually "heart medication"! :)
Forcing yourself to smile, may seem unnatural, but it can actually work well enough that eventually you'll forget why your forcing it. It used to amaze me how super attractive, intelligent, fit women stay with ugly, dumb, out-of-shape men because: "... he makes me laugh!" But if you think in terms of the heart, this make perfect sense. I truly doesn't matter how smart or attractive you are, as long as you can engage/touch and lift/lighten someone else's heart, they will respond in kind!
Here's a beginner's homework exercise, if you live in a city. Next time you see a homeless person or panhandler, smile at them say something kindly to them, even if you don't give them anything physical. When I've spoken with homeless people, they have told me that they often appreciate that as much or even more than money. In other words, though someone's body and mind may be starving, their heart is likely also starving for a light or kind word or look as this is often as powerful and in fact more meaningful than the meagre spare change they might get.
The most shocking thing of all, is that most city dwellers who are NOT homeless have the SAME heart problem! The only difference is that their bodies and minds are typically not starving, but there is a very good chance that their hearts most certainly are. For more advanced, optional homework, try breaking the old childhood adage of "Don't speak strangers". And, while I recommend some restraint, (ie: take care with too much energy, flirtation, etc. at first), try forcing yourself to say a word or two to random strangers when you notice something odd, funny and comment-worthy. Don't just keep "your inside voice" to yourself. Let the ridiculous and silly out into the world and see that what happens. Try thing "So what if..." rather than just "what if..." At worst, nothing will happen and at best, you may have a surprisingly interesting moment of interaction! :)
While "random acts of kindness" and "paying it forward" are fantastic, they take some working up to. Start small with simple "random words of lightness" and see where they take you! Good luck! :)