The iTunes connect keywords are the lifeline of every app developer. That 100 character field can mean the difference between a successful app getting a lot of downloads and an app that never gets any kind of traffic. The iTunes Connect keywords that you enter for your app are the simplest and most effective form of marketing you can do, so make sure that you don't make any of these common mistakes.
There really isn't a reason not to take up as much space as possible when inputting your keywords. Unlike in web search, Apple puts a lot of trust in you to select your keywords correctly. So make sure to take advantage of that and fill up all your keyword space with relevant terms.
In most markets, your app name and company name count as your keywords, and can be combined with your iTunes Connect keywords to form multiple word phrases that you will also rank for. On the other hand, if your app is in the Games/Puzzles category, it will show up for searches that include "games" or "puzzle" but not any combinations of "games" and "puzzle" with your actual keywords. So only apps that are very popular should consider including their category as a keyword.
Another common mistake is including spaces between keywords. Imagine Rovio is coming out with a new game named Angry Birds: Clay Castle, and their new intern comes up with some new keywords:
Wait up for a second, intern! You didn't bother coming out with good keywords, but at least don't waste space! If only you'd bothered to use our Keyword Optimization Module, you'd know that it makes no sense to have spaces or phrases as your keywords. Let's go ahead and re-factor those keywords:
We went from 78 characters to 47 characters. Of course, the keywords are still aren't fully optimized, as some of them just aren't common words people would use to search for an Angry Birds game. But at least they don't take up all the space!
This can be a bit more difficult to figure out, but some keywords simply receive a lot more traffic than others. You can easily rank as the number one app for "constitutionality," but you'll be the only one! Not a lot of people search for uncommon keywords, so make sure the keywords you're striving to rank for have a reasonable amount of traffic.
Sometimes being too ambitious can be a bad thing. Take a look at this example keyword list:
These are all keywords that receive a lot of traffic, meaning people search for them quite often. But the competition is fierce and brutal, and a new app developer, or even a fairly seasoned one with thousands of downloads, has no chance to compete with the really big apps with millions of installs. There is a delicate balance and trade-off between keyword traffic and keyword difficulty, so you must take great care to compete only for keywords that you can actually rank well for.
This might seem obvious, but long words are both less likely to be searched for, and take up more space. A nine character keyword for which you'd rank first could be replaced by a four and five character keyword pair that might potentially bring you more traffic.
This is a piece of advice we've all heard multiple times, but it only makes sense if the plural version of your keyword gets almost as much traffic as the singular version. In most cases, there is one common way that users will phrase something, and anyway your app will be ranked for the plural (or singular) versions of your keywords.
UPDATE: For a complete guide to choosing the best keywords for your app, read our complete guide to the keyword section process.