If you bring us some example lists it will be easier for us to help you.
Shooting in the dark, there are 5 main elements to most guard lists.
Heavy Weapon Teams:
Just 2 guys manning a heavy weapon, usually an autocannon. Shut these down quickly with long range anti-infantry stuff before moving your important vehicles into firing range.
I haven't had much experience dealing with these other than to hit them with template weapons & large blasts before assaulting them. Don't put infantry in their rapid fire range. Killing the dudes issuing orders may help?
These are usually put in front of tanks in order to chase away infantry threats.
Valkyries & Vendettas:
The hope here is that the other guy takes too many Vendettas and doesn't spend much on ground forces. In which case, reserve your fast stuff, don't be afraid go to ground when fired on. He'll hit really hard on turn 2 and again on turn 4. Try to be in cover, out of line of sight, or turboing on those turns. From there it's your job to win turns 5, 6, and 7.
If he's got only 1 to 2 flyers, play the game normally and ignore / out-manuever them when they're flying. Remember that often they'll hold scoring troops.
Vets in Chimeras:
Usually an expendable squad full of melta or plasma. The transport is cheap, armor 12, usually has a heavy flamer, and a multilaser. Really, I've got nothing useful to say here. It's nasty, tough, has answers to most problems, and the other guy will likely bring many.
Nightshields are your best bet here, as is taking allies with good long range firepower, since ours is pretty poor.
Leman Russes & Artillery:
Identify which ones will be causing the most damage, shoot dark lances at 'em. Clear away their infantry screen then assault them. Consider ways to close the distance, like Outflanking, Infiltrating, Deep Striking, etc.
Sometimes your best bet is to bring some attrition of your own and reserve your fast & fragile dudes. This lets you take down as many problem targets as possible before sending in the melee dudes.
Beast squads with buckets of Razorwing Flocks are kinda my silver bullet for helping to deal with gunlines. Your mileage may vary.
Time for the grateful DEldar.