1) Axii is the only sign you can use while riding (it defaults to it when you mount up, and back to your previous choice when you dismount iirc) and reduces roach's fear level when used
2) Axii can also be used on Alghouls to force them to lower their spikes, so you don't get damaged from attacking them
3) Axii used on an enemy horse will scare it into throwing its rider to the ground, setting up a nice insta-kill
4) speaking of insta-kills, a human enemy stunned with Axii is executable from behind
5) using Axii during conversations is all well and good, but be wary if you're trying to Axii 2+ people. it seems to be based on scripting, but sometimes the others will notice you trying to brainwash their friend and get violent
6) Igni drives away bees and cooks the beehive, which eventually drops to the ground and allows "honeycomb" to be looted from it. good for quick cash, or quick health
7) using 2nd stage Igni on an enemy carrying a shield will force him to drop it
8) Igni lights candles/torches/braziers/fireplaces/fire pits/forges/bodies, while Aard extinguishes them
9) use Igni on naturally occuring poisonous gas (methane?) to detonate it and damage nearby enemies. don't stand too close
10) Aard, in contrast, will simply disperse the gas. neither Aard or Igni permanently remove the gas; it most often eventually returns within 5-10 seconds
11) Aard used on fire elementals temporarily snuffs the fire surrounding them, killing some of their effectiveness
12) Aard stuns enemies with shields, allowing you to strike past them once or twice for full damage
13) Aard can destroy stalagmites in caves, sometimes revealing hidden loot/pathways. you can tell which ones are destructible via witcher sense
14) Aard/Igni/Axii knock flying enemies out of the sky and mounted enemies to the ground, and a knocked down enemy can usually be insta-killed
15) upgraded Aard is an AoE stun/knockdown. combo this with Yrden (which slows enemy recovery time) for an insta-kill frenzy
16) Yrden used on wraiths (or any enemy that disappears) doesn't just slow them, but forces them to reveal themselves for full damage
17) if you know you are about to enter combat, quickly activate Quen. your stamina will almost instantly replenish (as you are not yet in combat) thus allowing you to immediately use another sign after entering combat
18) basic Quen starts out only being able to absorb one hit (but has a max damage it will absorb) before it requires recasting, but later levels of the sign increase this. you can tell how many more times you can block by the little orbs circling your body. be advised: Quen dissipates after a time. if you're using a controller, it will vibrate once when it times out
19) upgraded Quen has a chance for knockdown when it ends. useful for the obvious, as well as grounding flying creatures
20) upgraded Quen will also protect you from environmental effects (and heal you if you have it upgraded) like fire, poison gas -- even those caused by your own bombs. which means you can effectively heal yourself by dropping a devil's puffball or dancing star, putting up the Quen globe, and standing in the affected area.