Here's a list of things I've noticed Americans doing that they (ok, we) tend to think makes us look Israeli... but that actually make us look like fresh-off-the-Nefesh-b-Nefesh-flight olim, or worse: here-for-a-year-on-a-gap-year-program Americans.
Disclaimer: these are great things to do if you want to seem Israeli when you're in America. Just not in Israel.
1. Wear wrap-around pants.
2. Call the New Israeli Shekel a "shek."
This seems to be slang popular among the Jerusalem English-speaking crowd, but I've never heard it from Israelis. The formal term for the shekel is "shach," short for "shekel chadash," which could be the source of this bit of Anglo slang, but "shach" is only used by newscaster-types. Say it with me, folks: they're called are sh'kalim.
2. Wear tzahal clothing when you aren't in the army.
Yes, I'll admit that I went on Birthright when I was 18 and bought the requisite army shirt. (Hey, it matches my eyes!) But in Israel, wearing army clothing means you're actually serving in the army. In fact, Israelis get so sick of wearing army clothes while they actually serve in the army that you would be hard-pressed to find any olive green in an Israeli wardrobe. So save that tzahal shirt as a gift for your friends back in the US. In fact, wearing basically any shirt with Hebrew writing on it, in Israel, is a decent indication that you aren't Israeli (unless that shirt has a cut-out neck and says "madrich"-- counselor-- on it somewhere).
4. Wear a kippa when you aren't orthodox.
My parents are very active members of a reform congregation in the US, but dress my ex-hippie dad up in the right clothing and he could pass as a chasid. I have literally never seen his chin. When they came to visit me in Israel last year, my dad decided to celebrate being in the Jewish state by wearing a kippa (yarmulke) all the time. Problem is, like a tzahal uniform, a kippa has a specific meaning in Israel. At the very least, it means that you are either on your way to a synagogue or shomer shabbat and shomer kashrut, so for my dad to wear a kippa while touring the country on shabbat... confusing.
5. Say "shalom!" to strangers.
My husband and I were recently in a national park when a couple walked past us, smiled brightly, and said "shalom aleichem!" We were not at all surprised when they turned out to be German Christian tourists... we would have been shocked had they turned out to be native-born Israelis. On the other hand, feel free to strike up a conversation with any shop owner, bus driver, or waiter that you see, and say "shabbat shalom" anytime to say goodbye to any Israeli you meet any time past Thursday morning. By Israeli standards, anyone you actually interact with for more than 30 seconds is no longer a stranger, so it's fine to greet them/share your life story.
6. Be loud, angry and combative.
"What??" you're saying. "Israelis are loud, angry and combative!" But here's the thing: Israelis are loud and combative, but they aren't usually angry. To Israelis, being loud and combative is all part of normal social interaction, and it's usually followed up with "shabbat shalom" and "tell Moshe I say hi." When Americans are loud and combative, on the other hand, we get angry, and we tend to leave in a huff with red faces and resolutions to never buy sandals in Israel again. As I said in another post, Americans are ruder (by Israeli standards) than we realize. If you want to seem Israeli, a better bet is to attempt to connect personally with whoever you meet. Being loud and combative is a higher level of Israeli-ness that we usually can't pull off.
I feel like there's more I should add to this list. Have you ever seen people on the street and just KNOWN they're not native Israelis? How did you know?
Then again, we American olim ALWAYS seem Israeli in America and American in Israel, so maybe we should just embrace it...