I find that a rediculously sharp serrated blade is the key -- I have a Wüstof bread knife with what they call a 'wavy edge' -- it looks more like a scalloped edge. I find it goes through bread much cleaner than a standard serrated knife. (if it has a soft crumb, it don't end up with the surface of the cut looking chewed up).
You also want a knife that's fairly long -- you want to take nice, long strokes with very little downward pressure; if you can, just blade slowly fall through the bread as you push it back and forth. Short strokes will end up with more of the 'chewed up' look.
I also find that the type of bread, and how you store it can be significant -- if the crust is too hard (eg, non-enriched breads), I find it more difficult to make a sandwich from it. I keep loaves that I know I'm going to be slicing thinly in a plastic bag, so the crust doesn't overly dry out (eg home-baked or those from the farmer's market they sell in paper bags or perforated plastic bags, I throw 'em in a plastic grocery bag when I get home, so it doesn't dry out in 24-48 hrs). If you have a bread box, that would work, too.
... and if none of those tips help, there are knives with 'slicing guides' available in a range of prices that have a parallel bar to help give you a reference so you'll slice more uniformly. To use them, you need a slightly different technique; as the guide protrudes past the blade, you need to make the last pass with only the tip of the blade.