Exceptional Tactics:
Welcome to the Tactics page of my Space Marine site. Here i will try to help you utilize your army to its best potential.
-Lord Dnigh.
Lets start with the obvious:
Marines have really good armor!
Lets see how we can use armor to our advantage...
1.) You can take the chance of utilizing open terrain.
2.) Wont die without taking a save throw... (Ap5 = average)
3.) Can take more shots without having to be cautious.
Okay, Another thing that works well for marines is sitting back and just pounding the incoming troops. This is most helpful against Orks,  Nids, and Dark Eldar.
If you plan to sit back, take low troops numbers, alot of heavy support, and Terminators. Also, you may want a squad (or two) of bikes, to come after the enemy gets to your lines and surround them in CC. This will prevent retreat, and a mobile choice to chase the fleeing troops down (if your playing a point battle, this helps because less points will be left on the field.)
One problem i have found with marines, is that they are easily outnumbered. The problem with this (sometimes) is they could outnumber you, and have the chance of killing more of your low initiative marines before they can strike back. A good idea to stop this is stay in terrain (always strike first) and have some mobile troops (an assualt squad or two) that can go and help out your engaged troops.
Another good tactic the marines can employ is the tactic of luring. This is very good against Nids, Orks, and speedy armies.
Simply keep troops in reserve, and when the enmy is near, drop pod the troops behind the advancing troops, creating a box around them. Be sure to have resilient troops fighting in the first couple turns on the enemy's advance so as not to run low on men.
"A single Marine is worth more than any man. Take care to treat them as such and you will win the day." -General Fox at the battle of Frontier. First major Victory for the Red Arms mobile Division.