On Match I got a lot of messages every week. I don't remember now how many, but it was over 100 a month. (I just don't remember how much over. A little? A lot? Dunno.) But I'm a girl, and that is what it is like to be a girl online. I wrote back very seldomly compared to how often I was written. I didn't meet every guy I write back. In fact, I didn't meet most of the guys I wrote back. I gave a very small percentage of guys my phone number to talk with them (fewer than one guy a month). I met most of the guys I spoke to on the phone, and fucked all but two guys I met. I never stood anyone up.
My female friends who used dating sites the way I did (to help weed out potential sex partners, which is easier and faster online than in bars, gyms, churches, etc.) reported similar stats. The only difference is I am a better judge of character than the women I know/I wasn't lying to myself/the men about only wanting sex. Or maybe I'm just pickier? Anyway, I fucked the guys I met on Match for years. They either lost touch after a few weeks or jumped into short-lived exclusive relationships with theirs. One of my very best male friends, someone I could have married if our values matched better, was a dude I met on Match. We actually talked about it for a little while, but realized marriage didn't mean the same thing to both of us. No other dating site introduced me to anyone else that compatible. I met the most attractive men on Match, the wealthiest ones on OkCupid, and the craziest ones on PlentyOfFish. A little crazy is okay if it's just for sex. Makes it interesting sometimes.
Anyway, I wrote back to only the guys who had few or no typos because typos indicated a lack of interest to me. I only wrote back to guys whose profiles were filled out, and who had lots of pictures. I preferred pictures of them out doing stuff. Anyone can say they are fun. Show me. You hike? Lemme see you in the forest. You bowl, let's get a picture of you releasing the ball. Whatever. Sure I just wanted sex, but sometimes I wanted to be exposed to new activities on the way to having sex. I only wrote back to guys who had something to talk about other than my impressive rack. I already know if dude wrote me he likes big tits. Let's not overstate the obvious. Instead, take all points of contact as an opportunity to display your fun personality. If a dude is no fun, I just can't get wet. I only wrote back to dudes who could work in some way to let me know they actually read my profile. I can't fuck people who don't like me, no matter how cute they think I am. If it wouldn't otherwise be possible for us to be friends some day, I don't need any dick from that guy. Sex is sometimes awkward and strange. People sometimes accidentally fart, or bleed, or fall off. If we can't be cool and laugh together, I'll never be comfortable enough to relax and actually come. Instead, I'll always worry about the little things that sometimes go wrong. Sometimes after writing back and forth a few times I might be on the fence about offering my number. Usually, at that point, if the dude flat-out asked for it, I'd give it to him (or offer to call him).
In summary: Use spell check, and re-read your posts. Show a genuine interest in the person-hood of your target. Show that you are fun to know, or at the very least, nice, but confident. Don't be vulgar without invitation. Ask for a number (but not right away). Include action shots in your profile.
My advice is from the perspective of a woman who was seeking sex. If you are looking for a girlfriend, this stuff might not work for you.