I've had mixed experience on it. I've used HOTorNOT and eHarmony (back in the day), and Plentyoffish and OkCupid recently. I'll speak mostly on OkCupid (it's the one I've used the most). There are four key things that will contribute to your success on OkCupid:

1) Your age

2) Your attractiveness

3) Your success relative to your peers

4) The effort you put into your profile

If one of these is off, it will probably affect your success a lot. I can't really help you with 1-3, so let's focus on 4.

Pictures: DOs: Smile in them. Take pictures where you're doing something you like are also a plus (e.g. running, comic con, sporting event). Taking pictures with friends is okay, as long as it's not excessive and it's clear who you are.

DON'Ts: Don't take your shirt off. Don't take pictures at the gym. I don't care how awesome your tattoo is, but don't post a picture of it. Taking pictures doing wine tasting/beer tasting may be alright, but you wasted/partying it up probably isn't a good idea. ABSOLUTELY NO MIRROR PICS. Seriously, I can't state this enough. Nothing screams "TOOL!" like a mirror pic. Bottom line is, avoid anything that makes you seem like a tool/douchebag in your pictures. For example, I ride a motorcycle. You won't, however, see me on it in any of my dating website profile pictures.

The rest of your profile: Be yourself. Spell correctly and use good grammar. Be unique, and specific about what you want. Be confident. That's pretty much it in a nutshell. Oh, avoid putting your income down. It screams "douchebag", and will only succeed in getting you gold diggers/putting off people that think you're conceited/full of yourself. Your job title should be enough.

Now you've got a good profile, and you're set to go! Like I said before, there are 4 things that will really affect your success. A good rule of thumb is that you need to be a good 3-4 years older than your target demographic before you start seeing real success (obviously there are outliers, but in my experience and in the experience of my friends, this has been the case, at least for younger girls). If you're in college, you probably don't need to be doing online dating(and probably won't have that much success on it, unless you're very attractive), but it's sometimes interesting to see the responses you'll get (in my experience it was mostly from girls who weren't in college). Once you're out of college (23-25 ish), unless you're super successful for your age or once again, really good looking, you might not experience a great deal of success. Reasoning: girls younger than you are in college and are dating other guys in college and girls around your age are dating older guys for the most part. If you want to date girls your age I suggest you maintain a network of friends from college/meet other guys your age and have them introduce you to girls. If you don't have friends, join meetup groups online. Also, meeting younger girls who aren't necessarily in college is also a possibility (although they sometimes go for the college guys, too). After you hit 26-27, you're pretty much home free.

Now, you've got one (or more) dates! Now what? If you've met someone online, and they're at least fairly attractive, on the first date it's pretty safe to assume you're not the only person they're dating (after the fourth or fifth date, this may change). Be prepared to answer the "why are you on <dating site>?" question. IT gets asked 75-80% of the time on the first dates I've been on. Also, be sure you have at least a good idea about what was on their profile. If their profile states several times about their love for teaching, and one of the first questions you ask is "...so what do you do?", you haven't gotten off to a good start. Be sure to find common ground, and LISTEN, LISTEN, LISTEN. Women love to be listened to. Find something they love, and just let them talk about it. In the end, just be yourself. If it works, it works, if it doesn't, it doesn't. Also, if you're dating multiple women at the same time, be sensitive to their feelings and if it starts getting serious with one, tell the other(s) the truth. To prevent this from happening, if I'm going on dates with multiple girls, I'll usually try to make a decision fairly quickly before it gets serious so that I won't string anyone along. I really hope this helps, and best of luck!

EDIT: I realize that it may come across like I'm saying motorcycle riders are douchebags (I'm not) but I'd rather spring that on someone after I've met them, than give them the chance to nix me based on that alone.