The most important thing about those tactical shooters is that you have to keep line of sight to your enemy, and shouldn't rush actions. For example, if you KNOW there is someone right around the corner, don't just go right around in the same turn. Walk/run to the corner, and stop just short of it. That way, you'll have some leeway if the enemy tries to go around it himself (Interrupt), and you'll have full Action Points when the next turn comes up.
Most deaths in those games happen because you run right up to the enemy in a turn, wasting all your AP, just to be shot to death next turn.
Also, line of sight. I don't remember JA2's key, I think it's ALT. That way, you can move while strafing or even walk backwards. That's important because getting whoever sees the other first has a huge advantage.