It seems that some of the whole grain loafs seem to dry out very quickly, even stored in a plastic bag - if it hasn't started to turn green, is there a way to restore some of the life back into them?
It depends. If you need it to soften up for just a little while you can throw it in the mircowave for about 10-15 seconds. And by a little while, I mean the time it takes to make and eat a sandwich. Other than that, you can use the bread for breadcrumbs or croutons. Also, dry bread makes the best french toast you will ever have. Basically, you can't turn back time on a loaf but you can make it last longer by freezing it and thawing a slice at a time.
Yes. Throw it in the oven on about 200 F for 20 minutes or so. The staling process is called retrogradation, and reheating can reverse it to some degree.
I have to disagree with the simple reheating method. Bread goes stale through a loss of moisture. Applying dry heat simply doesn't do much beneficial besides toast your stale bread.
Put the bread in a brown paper bag, dampen the top of the bag with water, and then microwave for a few seconds.
Cut it up and toast to make croûtons, or crumble into bread crumbs. Your bread will enjoy new life in a salad or other dish.
If your bread is coming pre-sliced and drying out quickly, try buying unsliced loaves and slicing it as you need it. It's a bit more work, but your bread will taste fresher longer (and as a bonus, you get to decide on the thickness you want).