Try approaching the slash at an angle of about 45 degrees rather, rather than at 90 degrees from the top of the loaf.  Also, set the blade at an angle of about 45 degrees to the surface of the loaf so only about half the blade actually enters the proofed dough.  You could try lubricating the blade, using water or oil, but I've never found that to work very well.  Also, make sure your blade is impecabily clean with no residue, no matter how small, dried on its surfaces.

For a fairly comprehensive review of bread scoring, see The Scoring Tutorial.

For very high-hydration breads, it's pretty much a lost cause to use a knife. Some find scissors work better. For most breads, what helps is:

1. A very sharp blade.
2. Wetting the blade.
3. Making your cuts with a firm, fast stroke. He who hesitates is stuck!

Do check out the tutorial.

David

that is a wonderful tutorial, thanks very much.

It's been said already and I will agree, firm clean and fast.  If you hesitate or if you are afraid of messing something up your blade will drag.  You also want to make sure of course your blade is extremely sharp and clean.  Even if you use a scalpel blade it might be dull so be sure that is not the case.

After years of baking I have come to the conclusion that good slashing is a matter of practice above anything else.  I use serrated table knives (like a butter knife but with serrations and with a plastic handle), it has gotten more dull over the years but works just as well for my slashing.  I think they key is to know how well your knife/tool works on the dough and using both the right amount of pressure and the right speed.  I've used a sharp knife also, but have better luck with the serrated knife because that's what I have the most practice with.