These have plagued me for a while, although I went a few months without getting any. Then last week I got one and I didn't catch it in time to prevent it. (I used to use just aciclovir ointment.)
Like others have said, vigorous kissing can bring them on - when I got mine I kissed vigorously at the same time I was going through a stressful time. The other thing that can bring them on is sunlight. I own an SPF lip balm but I neglect to wear it every day.
I have managed to keep it small but it's still visible.
I'm going to be prepared in the future. I will always wear my SPF balm. I will take Lysine every day. I already keep aciclovir ointment handy (in my wallet and my bedside table) but I have also ordered the aciclovir tablets and those will travel with me always. If I get to the tingle stage (or even if I miss it by a couple of hours) I will take the tablets for five days. I am also going to buy the infrared light treatment thing from Boots, as I heard that is effective and I will use that as it can be used as well as the ointment. I have heard that ice works but I rarely have that handy - you have to get it when it's still tingling. But if I can also get ahold of ice I will use that, too.