Strategy … what are you aiming to find out from the conference: some very general impressions about what is going on with functional innovation, something focussed on a particular language or languages, or on particular technologies? Depending on this is …
Plan … CodeMesh has 4 parallel tracks (a real hassle) but it's well worth making sure that you have a plan for what you most want to hear, reflecting your strategy. You should be able to move between individual talks, but that can sometimes be tricky because of synchronisation between different rooms, as well as the distances between rooms. If that's tricky, then you can always change at session breaks.
Another way of choosing is to go to the talk that you know least about. If you go to the one you know most about, that talk will probably spend 80% or more of its time telling you things that you already know. Of course, it needs to be one you're interested in …
Networking … a lot of what happens during the conference is outside the sessions, so do make sure that you make the most of the coffee / tea / meal breaks, and the evening sessions. Speakers are always happy to chat, so you can engage with them outside sessions, or follow up from any questions you ask in a session (scary, but a way of getting noticed). You can always email speakers, introducing yourself, and ask a question, if you do not feel comfortable asking a question after a talk.
Find an introducer, and have them introduce you to some people. If you are going with your PhD supervisor or another colleague or friend who has been there before, they should do the introductions.
Presenting your ideas … you're going to meet people who'll ask what you're working on and you want to interest them and move the conversation along, not stumble over how to explain what you're doing. So, have a 30 second elevator pitch ready.
Talk … OK, you may not feel ready for this, but if you had something specific and interesting to say you can often give a 5 minute "lightning talk" at meetings like CodeMesh. That get you and your work noticed, and people are usually very generous in listening and supporting speakers with less experience.
Learn … if you can get to the tutorials then you can learn a whole lot at these. There are some excellent tutorialists at CodeMesh this year.
Social Media … increasingly there is a whole virtual side to conferences, so track the twitter feed and other online stuff, and contribute yourself too.
Branch out … if you're there with your buddies, don't just stick with them, but aim to meet new people too, particularly at any of the conference social events. Even a casual conversation a couple of times over a couple of conferences lays a foundation for a deeper professional relationship, especially with peers.
Refreshments … there's always free (well, paid for in the registration price) coffee/tea etc. at the breaks and lunch in the middle of the day, but at quite a lot of conferences there's breakfast too, so it's worth getting along in time for that.
Afterwards you can follow up with people you have met by email or social media. You can also catch up on presentations which you missed by watching the video if the conference talks are recorded. Personally, I find that despite my best intentions, it's very unusual for me to find the time to do this.
Particularly for big conferences, it's best to pick and chose which talks to attend rather than sitting in on everything, otherwise you'll get burned out in the first couple of days. “I'd also advise listening to the speakers rather than sitting on your laptop/phone - personally I'd ban such devices from talks!” Also, keep away from alpha male superstars and their groupies … focus on the people who give presentations that fire you with enthusiasm, and those that take your presentation seriously.
Finally, have fun soaking up all the new ideas and meeting all the interesting people behind them.
Updates … thanks to Scott Fritchie, Andy Gill, Kathy Gray, Graham Hutton, Stefan Kahrs, Greg Michaelson, Neil Mitchell and Gem Stapleton for their comments and suggestions.