Pay attention to where the fish are and get your baits in a zone where they will bite it. When trolling on structure, fish use the structure to hide and ambush their food. Knowing this, it is important to keep your crankbait just off the bottom. Many times for this type of trolling I use no-stretch line (Berkley 10lb FireLine) so I can monitor the bait. I can let out enough line to just tick the bottom, reel up a little and I know my bait is in the “zone”.
For open water trolling, the right zone is often suspended. Many times the most active fish will rise up in the water column to eat and they will often feed in an upward direction. The best place to run a crank bait is at or above where the fish are marking on my depth finder. For this situation a good locator is critical because often open water areas are very expansive and you can waste a lot of time trolling in places where there are no fish.
Once you have located fish, then it is a matter of figuring out the right “system” for the day. How much line to let out, what speed to run, what type of crank bait, what color crank bait – all factors – but all secondary to finding the fish.