Comment, and I will consider adding you, based on the following:
Do I know you? I'm more likely to add people I'm already familiar with, either in person or online. If you're from an LJ community or other online group I participate in, and there's a chance I may not recognize you, mention in your comment how you found me.
Are you interesting? Even if I don't know you, I may add you if we share some of the same interests, or if you just seem like a fun sort of person whom I would enjoy getting to know.
Can you spell? I don't mind some typos or simple mistakes, but if your entries look like this: "OMGz!!!1 U'll nevar gess wut happn'd 2 me 2day!!!!!!" I probably won't add you. I'm rather persnickety about things like that.
If you still want to add me, by all means proceed and leave me a note. :)