It seems like common sense once you know but it really can make or break a hiking trip not properly lacing up your boots.
Mountaineering boots especially since they are so rigid and unforgiving. I was pretty impressed that our Ice Climbing guide from Yamnuska went over proper boot lacing. Basically, if your toes can touch the front of the boot, you're gonna have a bad time Ice Climbing.
If you tighten them up properly it should push the heel into the heel box of the boot and the boot should constrict at your toe knuckles allowing your toes to move freely. They will stay warmer as blood flow can still get to them and you won't get black toenails from kicking ice or steep downhills.
Another tip is a better way of tieing your shoe laces that I've learned.
You can see it here: http://www.youtube.com/watch?v=TKWL1md7Dv4
If your laces loosen after a while it might be because you are tying a granny knot instead of a square knot (I was doing this). The improved way of tying your laces in the video forms a square knot, it's quicker and I've found it much easier to tie with gloves on and with frozen laces.