Guide of tips and tricks.

A while back on 40k Online I posted some tips and tricks the thread didn't quite go the way I wanted and so I've decided to import my work over here so I can locate it more easily.

Name: -
Opponent: Any
Situation: A squad of Howling Banshees assaults and eliminates a unit in cover.
Tactic: If the squad comes under heavy fire going to ground becomes a good option (especially if the target you just removed was on an objective) whilst this isn't anything special what makes this tactic is the inclusion of Fortune and Acrobatic. A re-rollable 3+ cover save makes them quite sturdy and the inclusion of Acrobatic means the girls don’t care if they get assaulted. Taking the Farseer along with the assault also has some benefits.
Notes:  Whilst any unit will gain from Going to Ground in cover it is the Banshees that will gain most benefit, they can take a lot of fire and your opponent is going to think twice about getting too close or assaulting as they have counter attack and Fortune going, it can create a nice hole in your opponents battle line.
Harlequins also benefit very nicely.

Name: Chasing down Combat Tactics
Opponent: Vanilla Marines
Situation: Marine squad chooses to use Combat Tactics to fall out of assault range.
Tactic: If in close proximity several Eldar units have a move in the assault phase, which can be used to jump within 6” of the Marine Squad and force them to fall back further. Warp Spiders are the best unit for this as they generally have a larger bounce move but any Eldar jetbike can do this as well. Just make sure not to expose yourself unless the risk is worth it.

Name: Grav-Tank Manoeuvres
Opponent: Any
Situation: Using grav-tanks to control the field in the movement  phase.
Tactics: 
1)   Tank shocking units to herd them for flamers.
2)   Tank shocking units to pull it closer to your assault units
3)   Tank shocking to push units away or force them to move around the tank
4)   Blocking movement/assault paths or to grant cover saves
5)     Positioning the grav-tank directly behind a unit (that is spread out wide 4-5+ models) that is falling back. They must fall back by the shortest route so the grav-tank will wedge them a part breaking the unit coherency preventing them from regrouping later, saving time baby sitting them off the board.

Name: Dire Avenger Clipping
Opponent: Any
Situation: Dire Avengers shooting at an enemy squad that uses Rapid Fire weapons.
Tactic: This is a simple one that comes up fairly regularly. I see a lot of opponents spreading their troops out in lines, so obviously what you want to do is try and position it so your whole squad is in range to hit 1-2 of the enemy squad and positioned away from the rest of the unit. This will force your opponent to do one of two things, they can either stand and shoot getting a few additional shots from the squad members the extra 6” away in which case you will be the one putting out more offensive power (20 odd shots to your opponents 6 or however many are in range). Alternatively your opponent will move forward towards you to get into rapid-fire range only to find the majority of their squad is out of range and still offering minimal fire towards your unit. When this happens step back 6” whilst trying to position to clip again.
Clipping from multiple angles – This can be used against the Rapid Fire weapons squad or also works against any slow moving assault units (Khorne Berserkers who’ve lost their rhino for example). Try to clip the unit from opposite sides; your opponent will be forced to run towards one squad in particular so now reposition and repeat. This may mean, depending on their run roll and how close they are, that it may be beneficial for you to move towards them and charge or fleet away whilst using the second unit to shoot them in the back.
Notes. I have gotten both of these to work in several situations my opponents having cottoned on after 2-3 turns and whilst it may not seem like much that is 2-3 turns of one sided combat. When I say from multiple angles I don't mean 180 degrees because if the unit you are shooting at gets a good run roll it may put your second unit out of range so try and aim for something like 135 degrees. This allows you to be closer to each other for support whilst both being far enough to a particular side to clip successfully.

Name: 
Opponent: Orks 
Situation: Mechdar fighting an Ork army with Deffcoptas
Tactic: Not much of a "tricky" tactic but a realization that Deffcoptas are prime targets to Tank Shock especially if the ork player scouts them close to you first turn. 
Notes: Needing to pass a Leadership test before even thinking about Death or Glory makes this great, if theres a git w/o a rocket on the deffcoptta even better and if not at least you'll get a cover save if you go fast enough. Just dont expose yourself foolhardily.

Name: Double chances to sweeping advance.
Opponent: Any.
Situation: Charging into CC with a unit with an attached IC.
Tactic: This is quite situational really. If the unit you are charging is likely to be broken and you want to increase your chances to catch it in a sweeping advance you can split off the IC in the movement phase and assault it with both units. With a few exceptions (Exarch powers, psy powers, Apothecary etc) ICs gain little from being in the squad and now if you win the combat and your opponent fails leadership and is forced to fall back both the squad and IC have a chance to catch them with a sweeping advance. Best used against other fast armies CWE/DE or SM if you don't want them getting away and leaving you in the open. The drawback to this is of course the IC cannot rejoin the squad until the start of your next turn so s/he is vulnerable to being shot and assaulted separately though there may be times when thinking outside the box may be useful and I imagine if the IC and squad could join up in the consolidation move that this would be common practice.
Notes: This came to me one time when I had previously split Jain Zar off from a squad of Banshees, both units a turn later converged onto a single Grey Hunter squad when it hit me.