Lets be honest: conferences exist to waste time. Nothing productive gets done on conferences and best thing you can take our from them is to socialize with like-minded people.
1. Decide up front what sessions I am going to attend. All of these things should be stuff that I know very little about or take my knowledge from 200 to 300. Don't go to sessions that you know the answer to. Be super selective on sessions as well: skip vendor shills and affirmative action placements.
2. Look AND SCHEDULE casual between-session coffee meetings with folks I know are attending. This both deepens my relationships with them and you also tend to meet people most effectively this way that happen to be with that person.
3. Go to birds of a feather (if possible). People are much more likely to want to meet other people at these things.
4. Ask everyone you can "How their conference is going?" and "What are you working on in this space?". I make it a goal to ask at least ten people this a day between the coffee station lines, lunch, happy hour. At a great conference people love talking about what they are passionate about.
5. Drop out of late night events and drinking sessions if you are just there for the music or the beer and not getting knowledge or connections out of it. This is business, not party time.
If you can't do these things, yes, it is a waste of time.
But if you can, I find that I can leverage all of the info and contacts out of this relentlessly at my $DAY_JOB to get a lot done the rest of the year.