Here is a list of things I need to remind myself of during combat.

Facing:  +4 to hit back, + 2 to side

Relative Speed:  What speed is the attacker/target moving?   

Relative Size:  I compare the HF ( see Size Change) and give a bonus/pentalty based on the difference.

Distance:  Penalty based on how far away.

Called shot: -5 for limbs,  -10 for spots on limbs

Blind:  If you can't actually see the target, but have a good reason to think you are shooting at the right spot -5*

Parrying: Special rule with certain weapons.

Changing targets: a cumulative -1 on each attack in a multiple attack for each new target after the first.

Kickback:  a cumulative -1 to hit with non automatic guns after the first shot.

Grabbing:  +4 to hit if you are grabbing your target

Holding: + 8 to hit if holding ( restraining) your target, and they can not roll.  Keep in mind, they just might have a bonus to hit you as well.

Solid Hit:  If you rolled ten or more better than you needed to hit,  the target can't roll as the hit was so well placed.

* Sometimes the target is invsible, but you can still see their footprints; fighting in the dark, but you can still hear their breathing; its foggy but not totally dark;   camuflauged but not totally,