Many bait stores carry only the smaller pan fish leeches.  If these are all that is available you are better off using a different bait.  Look for the large black cold water ribbon leeches to increase your catch output.  

When hooking them you want to select a hook that is not too large.  You should be able to visually match the hook to the size of the bait.  You will hook the leech once right under its sucker cup.  When you keep leeches it is imperative to keep them in very cold water.  This puts them in a dormant state which will help you in two ways.  They will be very inactive when handling and will not try to attach to you when hooking them.  They will also stay alive much longer.  Once you place them into the warmer lake water you will notice they will become quite active rapidly.
 
When leeches are not available you can use whole nitecrawlers with excellent results.  Hook them once or at the most twice right neat their nose end and fish them as you would s leech.
 
If you are seeking only a trophy class Walleye you might want to switch to a large live sucker minnow.  This lake holds many Walleye that reach trophy class. These fish however feed on much larger offering than a leech more often than not.  Most bait shops in the areas carry live sucker minnows that are six inches long or larger.  Hook one of these once through the mouth and allow it to swim as freely as possible near the bottom.  You will of course need a heavier setup to fish for Walleye of this size.

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