Over the last couple of years of learning After Effects, I have found some pretty useful expressions which can help speed up your work process and create more seamless and smooth animations.

Here are some of my favourites.

Bounce Keyframes

Place on the position keyframes to create a bouncing motion.

// Inertial Bounce (moves settle into place after bouncing around a little)
n = 0;
if (numKeys > 0){
n = nearestKey(time).index;
if (key(n).time > time){
n--;
}
}
if (n == 0){
t = 0;
}else{
t = time - key(n).time;
}

if (n > 0 && t < 10){
v = velocityAtTime(key(n).time - thisComp.frameDuration/10);
amp = .05;
freq = 4.0;
decay = 2.0;
value + v*amp*Math.sin(freq*t*2*Math.PI)/Math.exp(decay*t);
}else{
value;
}

Loop

loopOut("pingpong")  // ABCD(CBABCDCBA)

loopOut("continue") // ABCD(EFGHIJKL)

loopOut("Cycle")  // ABCD(ABCDABCD)

Position one layer between two others

Place this expression on the 3rd layer (of three) to create the middle place.

(thisComp.layer(1).position + thisComp.layer(2).position)/2

Make a 3D layer invisible when turned away from the camera

Place on the opacity value of the 3D layer

if (toCompVec([0, 0, 1])[2] > 0 ) value else 0

Constant animation over time

Useful for rotation animations, fractal noise etc

time*100

See our first selection of expressions here

Hope these are helpful! let us know if you have any others...