Vitamin C has been doing my lips a world of wonders!
Please don't use that lemon juice crap. Yes, citric fruits have vitamin c but you're ultimately putting citric acid on them which is not powerful enough to effectively reverse sun damage and will only make you more sensitive to the sun and thus more sensitive to hyperpigment.
Use vitamin c on your skin. Get some L-Ascorbic acid powder, make your own little Vitamin C serum or toner, and use it on your skin every day. You'll see dramatic results even in just a month.
If you aren't used to vitamin c, start out making 5% solutions. Work your way up to 20% and go no higher!
I bit my lips a LOT this year due to them flaking and thus had crazy darkening. Even a couple spots were really bad. But now after making my own vit c toners, my lips are actually the color of my face and putting on foundation on my skin no longer makes me look like I have smoker lips.
Just use pure vitamin c on your lips. You can do chem peels too but I think vit c is more gentle since you're doing this daily and vitamin c stays in the skin for about three days. Building up the vitamin c in your skin will do wonders for your complexion.