A more comprehensive post can be found here.
Main point
Spend most (if not all) the time networking. Reason: meeting with people is the main feature of a conference, and the only thing that is hard to manage otherwise.
Every other conference activity should facilitate networking.
For example, I listen in technical paper sessions not because I have to (it is more efficient for me to read the papers at home); rather, I just want to compliment and chat with the speakers after their presentations.
I also like to see stuff far away from what I usually do, like animations and artistic sessions. I have met some very interesting people there, and got some useful inspirations for my work.
Below are more detailed points.
What (and what not) to say
A simple rule of thumb is to say only positive things. It can hardly go wrong. Humans like to be complimented, and I have never failed to hook up conversations with authors by saying how much I like their work. If you want to engage the conversation further, ask open-end high-level questions, like potential future works. Questions about details tend to make authors retreating inwards for answers rather than outwards towards you.
Demography
It is part of human nature for older people to like to talk to younger people, males to like to talk to females, and mortals to like to talk to nice-looking or charismatic folks. So, if you are a minority relative to the aggregate conference demography, e.g. nice-looking young female attending a computer science conference (which tends to be attended by people who are not very nice-looking, not very young, and not exactly female), you will naturally get more attention. Use this to your advantage. See that 70 year old Turing award winner standing in the buffet line who seems quite intimidating? Walk to him and ask about advices, and you will be surprised by how nice and talkative he turns out to be.
Conference size
If the conference is small, like no more than a few hundred people, try to talk to everyone. Most conferences belong to this category. Personally, I like these small venues because they feel kind of cozy.
If the conference is large, like SIGGRAPH, then it depends on your demography (see above). I usually try catching up with everyone I know first, and if time permits, knowing new folks that I find interesting.
Below is a rough sketch of my algorithm:
function UponBumpingInto(Guy guy)
if(I am already done talking to guy)
do everything to avoid another long talk without being impolite;
else if(I already know guy or guy is important)
chat with him/her;
else if(guy seems funky and I am not in a rush)
chat with him/her;
else
appear to be rushing to a session or occupied by an important phone message;
endif