Disclaimer: I do NOT condone cheating/sneaking your way into a paid event and will not be held accountable if you get caught attempting to do so.
Step 1: Identity...
Find a trash can/recycling can near the exit. When leaving events such as these, people are very likely to throw out their ID badges as soon as they are done with the event. Wait for the crowds to thin a bit and do some dumpster diving.
The badge will be found most likely in two forms: pin or around the neck. Figure out which it is by looking at visitors.
You may also want to keep in mind that there may be (for the conventions) a freebies bag. People are likely to put their badge inside the bag before throwing it out, keep this in mind and search the bags you find in the trash.
Once you have secured a badge, you may be worried that the person's name may be well known. To remedy this, slightly obscure it with clothing and walk inside.