6. Cover is your Friend
Almost every standard-issue weapon in the 40K universe, with the exception of the Lasgun (the Imperial Guard Mark III Flashlight), has the armor-penetrating capacity to ignore the nice Flak armour that the Imperium was so thoughtful and generous to issue to everyone in the Guard. Virtually every special and heavy weapon ignores that 5+ save, as well. Because of this, you will quickly find that your Guardsmen have a much higher survival rate if placed in or behind cover of some kind. However, you cannot let yourself succumb to the temptation of ignoring any of the previous bits of strategy above in favor of placing your Guardsmen into cover.
Guardsmen die. Even in cover they die and they die a lot. If you want to make their deaths mean anything (and do well when playing the game, besides), then you MUST ignore the cover available to you if it hurts your ability to concentrate your fire, breaks up your line of sight, makes it difficult to screen your units or respond to potential enemy assaults, etc. On the other hand, cover offers a great number of advantages, and if you can reasonably justify setting up your units in and around cover, by all means do so!
Cover has a number of inherent advantages that you should always keep in mind and exploit. If your enemy didn't bring his Frag or Plasma grenades along, your Guardsmen have first attacks in the assault (and if you're in Tight Formation, that means that EVERYONE gets to try and hurt the bad people attacking you...). If you're in cover, you get a saving throw against virtually every ranged weapon, with the exception of those with Flamer templates (which is why Guardsmen hate flamers). And most importantly, if you're in cover (and take care to set up a fraction of a inch or more from the edge), that typically means an enemy has to worry about difficult terrain. Bikes DO crash one in six times. Cavalry and Jump-pack troops break their legs and necks one in six times. Your opponent may not like it, but Guardsmen have no real hope of winning a "straight-up" fight. You have to take advantage of this Cover thing and milk it for all it's worth.