I always use one of those lit magnifying lenses to show up the faint red line of the blood vessel. I firmly hold the foot and toe that is going to be trimmed. I cut well below the end of the blood vessel and work my way up. 
I use convex toenail clippers on the birds, it seems easier to see what I am doing. I always have an open jar of Kwik stop (stypic powder) open when I work.