Even if you have an army that is built for the assault play it safe and do it smart. One example I was playing a Blood Angel on Blood Angel slugfest. We both had fairly similar armies he had 4 DC, 3 land speeders, and an attack bike. I had a Baal predator, a furioso, and 2 DC. It was really funny that we were both using the same character (Lemartes). Back to the example.
I took advantage of the game early on. I rolled to pick deployment meaning that he would have to deploy first letting me react early and that eventually won me the game.
After deployment I won the roll to go first and instead of rushing forward I just set up a more secure position because of my lousy deployment. He went and like the average player rushed forward hoping to kill me, but each and everything that he tried didn't work, not only leaving me with a Baal Predator, but with an entire counter attack, I had set up just right to swing my entire army within shooting distance and charging distance.
At the end of the game the only survivors were 2 assault marines, my Lemartes, the Baal predator, and the dreadnought's drop pod. Even though it was a narrow victory I think that the only ways that I could have won is my predator not dying and my devastating counter attack. So if it is an assault army versus an assault army try to be the one that counterattacks.
If you are against the shooting variety then try to out maneuver (yeah right don't even try this against Eldar [stupid space elves]) give them as few targets as possible but you may need to give them some to get into their lines. Don't be afraid of death! You may lose a few of your men, but they will lose many of theirs, if played well. Reevaluate what your standing the game is. Are you winning, loosing or is it really close.
Only take risks were you need to. Also only take a risk that is absolutely necessary, if you are winning wait for your opponent to make a mistake. Always make them do what you want. Make them think that they have control. Always give them that feeling.
Remember to play it the smart way. Keep your opponent on their toes, but feeling secure. Make them think that they have the game but be able to take it from them. Never let them kill more units than it is necessary. Take the initiative and own the field. You need to control the distance and tempo of the game. Thanks for keeping yourself awake through another one of my boring articles. Happy Gaming!!