#1: Lose Some Weight
Let's face it, if your on an online dating site there's a 99.9% chance you should lose some weight. Maybe you're not so fat that it's a health issue, but if you want to increase your success in online dating, or just life in general, losing some weight will go a long way to that end.
Now I don't want to give an entire lecture on how to lose weight, it's a complicated subject, but the two most important tips I would give anyone who is not having success losing weight is to be happy and lift weights. Easier said than done of course, and it probably involves getting outside your comfort zone and going out and meeting new people, but once you are happy you will no longer need to turn to food for happiness and your urge to eat will decreasedramatically and what seemed like an impossible task of eating less will become second nature.
#2: Have a Positive Profile
Do NOT whine and complain. Women are especially guilty of this. "Don't message me if you have shirtless pics". "Don't message me if you're just after sex". "Stop sending mean messages if I don't respond, it's nothing personal".
While these are valid complaints, first impressions are everything. If the first impression someone has of you is that you are a whiner and complainer their attraction to you will plummet dramatically. You're not special and you're not the only person that experiences these negative aspects of online dating. Your profile should give the impression that you are a fun and happy person, so only talk about fun and happy things.
#3: Be Unique
No, listing your favorite movies, TV shows, and music doesn't count. No, including the fact that you enjoy food, and like having fun doesn't count. No, stating that you can't live without water and air doesn't count.
Share something that is specific to YOU. Share a short anecdote about something that happened in your life. Not saying you can't talk about your favorite things, but NOT it list format. Talk about how you and your family sat together ever Thursday evening to watch your favorite show, how you went to see the band in concert and the crazy thing that happened while you were there, or have you have this funny personality quirk that makes all your friends laugh. For a person to be attracted to you they MUST be able identify with you specifically, or else you will end up like an extra in a movie that nobody remembers.
#4: Don't Write Formally
Whatever you say in your profile should be said in the same manner you would when you talk with a close friend. Fun, fluid, and free of thought about what anyone else might think. It shouldn't sound like a speech you stayed up all night to write. Looking like you're trying to hard and taking dating too seriously is one of the quickest ways to kill attraction.
Of course that isn't to say you shouldn't put effort and thought into wiriting it. It is a resume of sorts afterall. You want to present yourself in the best light possible which will take more than 5 minutes, but don't go overboard. Use mild slang, informal speech, laugh, and make jokes. If you aren't sure how it sounds, read it off to a friend and if they look at you funny then you need to relax and rewrite it more casually.
#5: Send Unique Messages
I know there are lots of sites out there that recommed copy and pasting messages to hundreds of women, and that may work if you're a really good looking guy who is just looking for sleep with anyone who has low enough self-esteem, then sure, go ahead. However, if your goal is to actually me someone who might actually have similar values and interersts, then spend a few minutes looking at their profile and pick something to start a conversation about.
Again, the short anecdote works wonders. "Oh hey, you like that band most people haven't heard of? I went to see them in concert in the past one time and a funny thing happened. Man it was crazy! Have you ever had something crazy happen to you?". Doesn't that sound like someone you might want to talk to? At least a lot more than "Hey you like music? I like music too. Do you like other music?".