Useful Tips for the Average Player

Please note the aim chances are rolled once the turn starts, so loading the same save and doing the same actions gives always the same results. You may get a different roll by either changing the attack order or moving.

Originally posted by Lightna Core:

Another thing to note on reloading saves may change the positioning of alien spawns or what may exactly spawn at that spot. Yes, the Hyperwave Decoder does tell what aliens are going to be present but if you stumble across a Sectopod then reload a save, you may get Mutons instead in that particular spot while the Sectopod will spawn elsewhere.

• How to deal with panicked soldiers: Unless it risks other units' lives, fall back and wait until it recovers sanity. Sometimes its not worthy to push forward while one or more troops are panicked.
  
• How to deal with panic levels: They should always be your top priority when choosing abduction missions, being the others (in respective order or priority) new recruits, engineers, cash and scientists. As a general rule, allowing panic levels to raise will make missions harder, so save your best troops for such emergencies. Losing a country is no reason to ragequit, since you only lose the game if you lose eight countries! On classic/impossible mode, you will often have to neglect some countries to keep more useful ones. In my opinion, Europe is the least important one (sorry europeans!).

Other than firing at their backs, you can also throw explosives to destroy cover and use psionic powers such as mindfray, mind control and rift (mp only).

Second Wave Hint: If feeling confident enough, you may turn on Absolutely Critical, which guarantees a successful flanking hit to be critical (but not the hit itself), regardless of it being from you or the enemy.

Originally posted by Traul:

A special tip for Classic: an overwatch shot from a sectoid cannot kill a 5 life soldier, a regular shot can. If you need to risk an overwatch shot to get a sure kill this turn, do it.

Originally posted by Dr Vahlen:

You may want to instruct your men to exercise restraint when using explosives. While certainly effective at killing aliens they also destroy the artifacts we're hoping to recover from their bodies. Just something to consider.

• Dashing gives your unit a dodge bonus against overwatch fire. Still a very risky move, only do this if you don't mind wasting a soldier.

• A safer way to fool overwatchers is walking through full cover. Even if the enemy spots you, he wont fire unless you leave your cover.

• Suppressing fire on an overwatching unit will turn off their overwatch. (credits to Raithul)

• An assault with sergeant rank has Lightning Reflexes available, guaranteeing him to dodge one overwatch per turn. Or more if they are triggered at the same time.

• A support colonel may learn Sentinel, allowing him two reaction shots per enemy turn. Not really useful in my opinion, compared to Savior (+4 healing on medikits).

• An assault captain has Close Combat Specialist ability, which gives him as many reaction shots as his clip allows with no need to overwatch, if the enemy approaches him by 4 tiles.

• On escort missions, you are going to face several thin men falling from the sky and then overwatch. If there are reaction shots still available, they will be used against them. Very effective with squadsight snipers. (Credits to Davy Jones)
  
• When facing melee units, retreating and overwatching is the best way to ambush them. They often don't think twice before rushing at your troops.

• Be careful if you fire at a muton berserker on your turn, as he will move towards you and possibly trigger any reaction shots you may have placed!

• Lure aliens into an ambush. If the AI knows you're there and retreating, they are most likely gonna chase you. This is a good method to deal with large groups of enemies.

• If a unit took too much damage, get it out of the enemy's sight, then wait for an opportunity to use a medkit.

• Fall back if one of your troops is mind controlled and you can't deal with the controller. Mind control has a cooldown of 4 turns, so either wait or, if it's too dangerous, kill your own soldier.

• Keep a distance of at least 9 tiles from chryssalids, so they wont be able to attack if they reach you. (Credits to amirhuman)

• Alien Base Assault: Carapace/Skeleton armor and laser weapons are recommended, although some people can do it with just the basic gear.

• Overseer Crash Site: Should be okay with the same sets as above, but light plasma rifle and alien grenades helps. Please note it actually starts by building the hyperwave relay, so a firestorm interceptor with a good weapon is a must to take down the overseer UFO!

• Temple Ship Assault: This is the final mission, so you most likely have the best gear possible at this point. Titan armor (or psi for the psionic soldiers) and plasma weapons are the minimum.

Pros:
• Unlocks interrogations, which reduces certain researches' duration by 50%.
  
• You can use the interrogated alien's corpse for autopsy or item crafting.
  
• The stunned alien's weapon won't break, allowing you to equip it on your troops after doing proper research!
  
• You may also get a limited supply of alien grenades by this method, before you research it and it becomes infinite.
  
• With the Foundry, you can use it to capture drones and repair SHIVs.

Cons:
• It occupies the backpack slot, which could be better used with grenades.
  
• You often need to damage the alien a lot before successfuly stunning, which may kill it if unlucky.
  
• The user needs to get dangerously close to the alien.
  
• Not recommended for "farming" weapons. If you really need better guns, craft them on HQ.

• Overwatch doesn't trigger unless a unit moves within two tiles while inside the Overwatching unit's line of sight. That means if you are caught in the open and stuck in an Overwatch trap you can move a single tile out of cover then another tile into hopefully better cover. To avoid said traps just move enough that you can enter a unit's line of sight but no farther, this makes Deadeye snipers in multiplayer trivial to defeat.

• Combat drugs gives you +20 to aim and +10 to crit, which can be devastating if your soldier is able to fire twice in a single turn.

• Press the unit info button (Or LS if using a controller) to view a unit's stats, buffs and debuffs. Useful to find out why your soldier has a red arrow next to it.

• Mind Merging a Sectoid Commander will give you an extra 25% to land a Psi attack on any unit, that means you can Mind Control Chrysalids and other SCs. In SP you can use Psi Inspire to buff your Psi Soldier's will, letting them MC Ethereals. Note that hitting an opponent with Mind Fray softens their will, making them easier to MC.
  

  Maybe I can get Xavier achievement after this tip.

  
  
• Grappling hooks can be used to avoid suppressions and overwatches entirely.

• And last, but not least, do not underestimate the phoenix cannon! It's almost impossible to take down your first large UFO without it, especially on higher difficulties!

For those looking after advanced strategies at high difficulties, I recommend reading this: http://scientificgamer.com/xcom-101/

Oh, why dont you have a look at this video? Its aimed at ironman impossible, but gave me valuable help on singleplayer in general.