There are several hand gestures that characters might use to express hope, wishing good luck or trying to avoid bad luck. It can be accompanied by idioms like "fingers crossed", "keep your fingers crossed for me" or "break a leg". It might indicate that the characters are superstitious, but frequently it's just a natural part of Body Language tropes. Mind that these hand gestures might vary cross-culturally.
FINGERS CROSSED: People cross their index finger and middle finger, sometimes on both hands. Generally it has the meaning of wishing for good luck or good fortune. Alternately, it may indicate intense hope or it can serve as anti-jinx measure. note The gesture has probably pagan or early Christian origins where the gesture was believed to ward off evil. Some scholars think that crossing your fingers is a way of making the Christian sign of the cross, and as such, it invokes a prayer or desire. This gesture is visually similar to Lying Finger Cross, but as its meaning is positive, it's not hidden.
THUMBS CLUTCHED IN PALMS: Characters firmly clutch their thumb in the palm, and it can be done with both hands. It serves as wishing good luck for somebody else, or it expresses that a character asks the others to wish them exactly that.
THUMBS UP: Thumbs Up is a common gestures made by extending the thumb upward. It has several meanings (OK, number one, hitch-hiking, showing a direction, or an obscene gesture). It usually means approval or praise. Sometimes it's used as support or wishing good luck. Examples from fiction listed here should convey just the 'good luck' meaning.
FIG SIGN: Fig sign is a gesture made with the hand and fingers curled and the thumb put between the middle and index fingers, or the middle and ring fingers, forming the fist so that the thumb partly pokes out. In some areas of the world, the gesture is considered a good luck charm; in others it's considered obscene.
KNOCKING ON WOOD: A superstitious gesture used to ensure that a good thing will continue to occur after it's been acknowledged. However, it's sometimes used after speaking of an unfortunate event, which is likely to happen, so that it does not actually occur.
JAPANESE "BREAK THE BOX" ANTI-JINX MEASURE: The forefingers and thumbs of both hands are pressed together to make a box shape. Another person has to "break" the box shape with a downward motion of the forefinger.
Examples from media:
Examples of Fingers Crossed:
Film - animated
In The Simpsons Movie, Homer hopes that Ned Flanders confesses to being homosexual. Rather intensely, too.
Ned Flanders: The Good Lord is telling me to confess to something...
Both versions of The Parent Trap have a special gesture. They cross fingers (for luck) on both hands, with arms crossed (symbolizing the girls' Twin Switch). It was used much more in the original Haley Mills film.
In Paycheck, after a subway train stops just shy of hitting Jennings, you can see that he's got his fingers crossed, presumably in a last-second plea for luck.
All the girls do this behind their backs in Madeline when an ambassador visits their boarding school, because they don't want him to purchase it. Pepito manages to scare him off with firecrackers.
In The Shining, the Torrances are driving up the mountain in their unreliable VW Bug. Danny is confident that the car will make it. Wendy isn't and keeps her fingers crossed, Danny glances down and sees through her sandals that she crossed her toes as well.
Live Action TV
Friends, "The One Where They’re Up All Night": There is a beeping sound coming from the living room in Phoebe's apartment. She sleepily goes to investigate, crossing her fingers on both hands, saying: "Please don’t be a space ship. Please don’t be a space ship." It’s a broken smoke detector.
Horrible Histories, "Terrible Tudors": In the sketch "Queen for Nine Days", Lady Jane Grey writes a letter to Mary Tudor, asking her to recognize her as Queen. When she hands the letter to be sent, she crosses her fingers on one hand and says: "Fingers crossed!" She smiles very intensely.
The Big Bang Theory, "The Holographic Excitation": Stuart says to Raj that he tries to use Halloween parties as means of meeting women. He crosses fingers and says: "The ninth is the charm."
In "Slutty Pumpkin" of How I Met Your Mother, Lily crosses her fingers before the results of Halloween costume competition for couples are announced.
Vanille from Final Fantasy XIII does this whenever she is worried things might go wrong. She does it a lot of times throughout the game.
"Brother, Can You Spare Two Dimes?": Homer crosses his fingers on both hands when he's expected to receive the First Annual Montgomery Burns Award for Outstanding Achievement in the Field of Excellence.
When kids of Springfield Elementary take a career aptitude test and are getting their results, Martin crosses his fingers and chants "Systems analyst...systems analyst" before getting the result of systems analyst.
In "Missionary: Impossible", Lisa Jr. crosses her fingers when she says she wants to give up gambling. She doesn't hide it behind her back so her wish is probably sincere and she really hopes to stop.
In the Centurions episode "The Sky is on Fire", Crystal tells Lucy the orangutan to do this just before attempting to teleport a weapons system through atmospheric disturbance. Lucy holds the gesture behind her back.
In The Smurfs episode "The Blue Plague", Papa Smurf tells the prince-turned-frog to cross his fingers for luck as he mixes up the poultice that will restore him to his human form. The prince tries to cross the fingers on his hand, but since he's got the hands of the frog, the fingers won't stay crossed. Papa Smurf laughs and excuses himself for that statement.
Examples of Clutched Thumbs:
In Warhammer 40,000 Imperial troops sometimes "thumb their palms", a gesture that resembles the aquila, the double-headed eagle used as the Imperium's insignia.
Examples of Thumbs Up for Good Luck:
Film - Live Action
At the end of Battleship, as Hopper goes to ask the Admiral for permission to marry his daughter, she gives him a thumbs up.
At the end of Terminator 2 Judgement Day, the T-800 gives John Conner and Sarah Conner a thumbs-up as he's being lowered into the steel.
In Lemonade Joe, Mr Goodman gives Lemonade Joe a thumbs-up and a supportive look when he goes to a final show-down with Hogo Fogo.
The Doctor gives Amy a double-thumbs up before she acts as a distraction in the Doctor Who episode "The Pandorica Opens", Amy gives a reluctant one of her own back.
Episode "The Execution" of 'Allo 'Allo! has an example which is probably a mix between thumps-up for good luck and signalling that everything is ok, and crossed fingers gesture is used too. René is about to be shot by Germans, but he was promised that they would use wooden bullets. Hans however brought two boxes: one with those fake ones as well as real bullets, and Lt. Gruber accidentally took both boxes. However, Colonel and Captain smile at René and give him a supportive thumb-up. They are shown to have crossed fingers behind their back, meaning they either wish the plan worked or that they acknowledge their reassurance was not sincere.