Here are some tips to help you increase your chances of getting your game/artwork/cosplay/film/whatever featured on Kotaku. It's not a guarantee your stuff will turn up on the site; even the nicest email can still be for a terrible game. But it'll at least help your chances by having us not ignore it/hate you forever.
Note: these are my personal tips based on my personal experience as a Kotaku editor for over six years. They're not a general guide for all of us, and I'm not speaking for every other writer here, just me.
1) Don't spam us. If you send an email and we don't reply (or feature your stuff), it's totally cool to re-send it. Maybe a day or two later. We might have been busy and just not seen your email, because our tips line is flooded with literally thousands of emails a day.
Sadly, we pass over most stuff because we simply don't like it enough to feature it. Sorry. As much as we'd love to reply personally to people telling them that, we just don't have the time or resources, so if you haven't heard back from us after a few days, you probably missed out.
It is not cool to send us an email every day complaining about this. Or, even worse, to get your colleagues/friends to send us 100 emails a day. That's spam, spam is annoying, and if you annoy us, you end up on email filters. Of which some of us have many.
2) Tailor your approach. Your email might get missed if it's sent just to tips. It can help to see which author covers what kind of stuff and appeal directly to them. If you're a musician, send it to Kirk. If you're a cosplayer, send it to me. If you're a comics artist, send it to Evan, etc. We've each got particular specialities and interests on the site, and it can help appealing directly to them. Same goes for gaming genres. Take a week or so and see who covers what kind of game.
3) Use Twitter sparingly. Where email is the equivalent of leaving something in our mailbox, Twitter is like tapping us on the shoulder while we're working. It can work (and work quickly) if your stuff is good and is the kind of thing the writer you're reaching out to digs, but be careful; annoying emails are bad enough. If you fill up a Twitter feed with spam, you will make people cranky.
4) Don't message our general Kotaku twitter feed or Facebook page. It's tough staying on top of the Twitter feed, and impossible to monitor the Facebook one. If you want to get in touch, use firstname.lastname@example.org or our personal contacts. Otherwise we probably won't see your tip/message.
5) Get your timing right! Let's say your game/art/project does appear on Kotaku. Congratulations! But this does not make us your personal, free PR machine, who will post about every new piece of media or every news update you have. Most stuff gets one article and that's it, sadly, so if you're an indie developer or someone running an event, ask us when you've actually got something to show, or when your Kickstarter campaign has already started.