Before I start, I just want to clarify that I'm by no means an expert at all this. I've only been cosplaying a year and I still have a lot to learn, but my tutorials and cosplays get positive responses and I'm often asked how I do certain things, so I decided to put up a few tips that have helped me get where I am.
1. Find a good cosplay photographer
I cannot stress how important this is. I've seen way too many great cosplayers who only have shots taken by their friends or family and it makes me sad their missing out on the attention they could get towards their cosplay if they just had some good photos.
How do I find a good photographer?
Pay attention to cosplayers from your area, whether it be Cosplay.com, WorldCosplay, Facebook or right here on DeviantArt. If you find someone who has a cosplay picture that you really like, ask them who did their photo shoot and ask for their contact information. If you're at a convention, look out for people with expensive cameras or who are in the middle of a photo shoot (remember to be polite and not bother them if they're too busy, they are working and the cosplayers are most likely paying for their time) and ask for their business card.
I did that, but they charge $XXX an hour! I can't afford that!
To me, good photography is just as important to your cosplay as a wig. If you worked really hard on it and want to show it off, you should add the cost of the photographer to your budget for the cosplay and save up enough to get good photos taken. If you pick a good photographer, you won't regret it and you'll have nice photos forever to remember your cosplay by, even after you're tired of wearing it and you've sold it.
2. Detail is key
The difference between a good cosplay and a great cosplay is all in the details. Nothing is too small or unimportant to be left out of the cosplay when you're making it/buying it, no matter how little you'll think it'll be missed.
For example, take two Grell cosplayers with the same outfit and wig. One of the cosplayers decided not to buy the teeth and contacts, while the other one did. They both look good, but the one who paid attention to the extra details will POP out at you more every time. I noticed this when I bought all the extra details of my Grell cosplay last (and again when I updated it a few months later) and I was shocked at how much of a difference it made to my face.
This concept is true to all cosplays and your character should be carefully studied. When you're making/buying your cosplay, always look to as many pieces of OFFICIAL art as possible (don't rely on fanart to have all the little details, no matter how nice it is). If you can, Google around for the character reference sheets. It often shows the character and outfit very clearly and at all angles. Yana Tosobos Pluto character sheet helped me out greatly in styling my Pluto wig since he has over 20 spikes on all sides of his head.
3. You don't have to be a jack of all trades to be a good cosplayer
One common misconception is that you're not a good cosplayer unless you can make everything yourself.
This is simply not true. Cosplayers at every level need help sometimes, if it be with wigs, makeup, props or even the outfit itself. Don't be afraid to ask a friend for help with something you know you can't do very well yourself.
I don't know anyone who can sew/style wigs/make props!
That's okay! Cosplay.com has a great Marketplace and forum where you can hire people to make something for you or you can buy something used (most of my Grell cosplay is various pre-loved pieces!). I recommend buying a used cosplay before looking towards a cosplay shop, simply because the quality might be better, the photos you see aren't misleading stock and the price is often much lower for something that's probably been only worn once or twice.
I'm on a budget, I'd rather learn how to sew/style wigs/make props/make up on my own. How do I get started?
If you don't have a friend who can teach you or you can't afford to take a class, the Internet is full of free knowledge. Deviantart and Cosplay.com have lots of tutorials you can read and /cgl/ (on 4chan) is usually happy to help if you have a question or if you want them to dump tutorials you might not have found anywhere else (/diy/ is helpful for props and /fa/ can help with more casual cosplay pieces, like a jacket).
Don't let cosplay websites limit you though! Youtube has lots of videos on how to do makeup and there's lots of non-cosplay websites on the various subjects.
I can't find any tutorials for the cosplay I'm doing!
If the Cosplay.com forums and /cgl/ have failed you, try doing a search for other cosplayers who did the character and ask them specifically how they did something or where they bought it. Ask as many as you can since some might be too busy to answer or others might have bought something too expensive for you.
4. If a good cosplayer posts a picture online, but no one is around to see it, does it make a sound?
If you want to be appreciated as a cosplayer, you need to get your work out there for the world to see. This isn't always easy for a new cosplayer, but there are ways to get noticed.
When you first upload your photo, be sure to add as many RELEVANT tags to the photo as possible (don't put things like rainbows, avengers, cute kittens!! in your tags to get noticed more unless your cat makes an amazing Tony Stark) so people can find it easier.
It also helps to have a good title so people know what it is right away. I always put the series name, the name of the character that I'm cosplaying (so people know which one I am without clicking if there's more than one cosplayer in the photo) and then a little title.
After it's been uploaded, feel free to submit it to groups so you can get noticed and get more watchers. I tend to submit mine to groups after I let my watchers finish faving it (usually a day-week, depending on how popular the photo is and if it made front page), but there really is no rule on when you should put it in groups. It's good to put it in groups that accept all kind of art as well as cosplay groups, groups specifically for the series you're cosplaying from, as well as any pairing groups or even specific character groups.
Once you've got a nice inbox filled with faves, always be sure to thank everyone for them! If you have more than you can deal with (I often let mine pile up to 1,000 messages before I start passing out thank yous. Don't be me, you'll regret it!), it's okay to use a cut/paste message for everyone.
I've found if you gently suggest that they watch you, the person who faved your photo is more likely to check out your page and give you a watch, which is good for you the next time you submit a photo. Don't be pushy or desperate when you say this though. Just word it so it's a friendly reminder that they may have just drag drop faved you and they might want to look at what else you have to offer.
Don't forget that you are not the only cosplayer out there trying to get noticed! Spend time looking at cosplayers and commenting on their work. It'll make their day and they might even return the favor and check you out too. A good place to find under noticed cosplayers is under the 'Newest' search option or 'Most Popular in 24 hours'. Just because they're popular today doesn't mean that they are all the time.
5. Only take on projects you can handle
Not all cosplayers have the skill or budget to be whipping out dozens of complicated cosplays every year, and I understand that. I even get envious at times when I see how many cosplays my friends and peers can do in such a short period of time, and it's discouraging to me as a cosplayer.
My rule of thumb is, if you can't afford it, don't do it. Believe me, you'll be much happier with the finished product if you take on a small/simpler cosplay instead of one you have to skimp on to make work.
But I /really/ want to cosplay this character!
Often times, a character will have more than one outfit to choose from. If his/her's default outfit is going to be more than you can handle, look into doing one of the alternate outfits. People will be impressed that you did something different from most of the cosplayers who do the more complicated version, and it sometimes opens you up to unique photo shoots.
You will always, always be happier with a few simple, accurate cosplays than one complicated, but lacking cosplay.
I hope these tips helped you out and improve you as a cosplayer. Feel free to ask me any questions in the comments below and suggest tutorials for me to make or other points for me to address. Be sure to have fun!