I realize that some of these things are due to a combination of factors, like available sets, and concerns that the TV audience might not be able to figure out stuff, but inspired in part by folks on Twitter discussing the so-called secret meetings two characters on a show I have not yet watched, plus, my own extensive TV viewing, here we are.
1. If people are not supposed to know you are meeting, do not meet out in public. Sure a busy restaurant or a bridge (people on "Nashville" seem to meet on bridges or other places with railings a whole lot) is a great place if your goal is for people to not overhear. But if your goal is to not be seen, you have failed.
2. If you are receiving secret phone calls, don't program the picture and name of the person who is not supposed to be calling you into your phone.
3. Corollary to the above, don't leave the phone on which you receive secret calls or texts lying around for people to look at.
4. People can hear you. I realize this is partly because having two characters take time to walk somewhere they actually couldn't be heard and then walk back to where they were supposed to be takes away from the actual time spent on the plot, but as a viewer I often can't tell if I should assume the person right next to the other person actually couldn't hear, or if this was going to come back and bite them. On "Leverage" people apparently could never hear them when they were talking into their earpieces, so eventually you accept that, but why not? It's not like all the people around them knew they were talking to an earpiece.
5. People with real secret identities never tell anyone. Seriously. Because once you tell one person, it's, oh that's right, not secret anymore.
6. The following do not actually count as disguises: Glasses. Hoods or hoodies. Hats. Seriously, people, do not do a worse job than Jem. (Who's boyfriend Rio, was admittedly an idiot, but not as much of an idiot as, oh, I dunno, folks who cannot recognize their children once they put on a hood.)
7. This would probably be too easy, but if you are aware you are in a TV show, you might want to befriend the writers. Because that seems to be the only reliable way to keep your secret.