I use a 180 Degree electrical wire cutter and a scalpel with disposable Blades for cleanup.
Also when cutting the parts off the sprue I check first for possible stress-points that might snap the part is cut off in the wrong sequence.
Either way don't excert too much force(ie, cutting of with a Knife, etc), rather cut closer to the sprue than the part, you can always clip the attached sprue easier of once the part is seperate.
For really tiny parts(1/35 Rivets, 1/24 hood clasps, etc) I either use tweezers or gently skewer them on the tip of a #11 Blade and position the part that way.