Look before sitting. Take a seat that splits the distance evenly between other sitters. Don't look directly at a person seated opposite. If possible, take a seat or table off-axis. Try to appear cool and indifferent, unless you're with someone.
Groups of people offer cover.
Teenagers often travel in packs. For that matter, so do older people. Notice how some people camouflage their identities- or try to. A man sleeping on a plaza bench or in a library is less likely to be disturbed by officials if he holds something, say a briefcase or umbrella. Police or security guards give tacit approval to some individuals while pressuring others to move along.